user26891: I'm curious what the purpose behind having a word list for BIP39 HD wallets? 
Are you required to use these words to create a wallet seed? 
What would happen if you chose random words not on the list, or instead a long continuous string?

user13866: 
I'm curious what the purpose behind having a word list for BIP39 HD wallets?

Its a carefully chosen list so that the words are distinct and hard to confuse. For example avoiding words that sound the same but are spelled differently (e.g. MAID and MADE) but also avoiding words with similar spellings (regardless of how they sound) and avoiding long words that people often have trouble spelling.
Having a fixed list makes it easy to identify accidentally mispelled words.

Are you required to use these words to create a wallet seed?

A BIP39 compliant wallet will insist the words be in that list. There are different lists for different languages.

What would happen if you chose random words not on the list, or instead a long continuous string?

If you try to use the wallet-recovery feature and type in words not on the list, your wallet would display some sort of error message.
When creating a wallet you generally do not get to choose the words yourself. People are very poor at choosing words randomly and this results in very weak seed phrases from a security point of view.

user7272: Technically, an HD wallet does not require a mnemonic. It simply requires entropy. The mnemonic system is designed to make human use easier, as it is far easier to write down a set of words, instead of trying to safeguard a long hex string.
The wordlists themselves are chosen to have specific properties, namely:

The first four letters are unique for each word across the entire list, which makes noting it down easier, especially with compact backup devices like a cryptosteel.
The list is sorted alphabetically, which makes lookup easier
The list avoids words that are similar looking and sounding, which makes it easy to correct improperly noted down words.


user5273: The bip39 algorithm is badly designed and that is why it is dependent on a specific wordlists. Each time you want to add a new language you have to update the BIP! 
Electrum has it's own seed mnemonic format that does not depend on specific wordlists so it's definitely possible to create a format like that. 

26891: <p>I'm curious what the purpose behind having a <a href="https://github.com/bitcoin/bips/blob/master/bip-0039/bip-0039-wordlists.md" rel="nofollow noreferrer">word list</a> for BIP39 HD wallets? </p>

<p>Are you required to use these words to create a wallet seed? </p>

<p>What would happen if you chose random words not on the list, or instead a long continuous string?</p>
 13866: <blockquote>
  <p><strong>I'm curious what the purpose behind having a word list for BIP39 HD wallets?</strong></p>
</blockquote>

<p>Its a carefully chosen list so that the words are distinct and hard to confuse. For example avoiding words that sound the same but are spelled differently (e.g. MAID and MADE) but also avoiding words with similar spellings (regardless of how they sound) and avoiding long words that people often have trouble spelling.</p>

<p>Having a fixed list makes it easy to identify accidentally mispelled words.</p>

<blockquote>
  <p><strong>Are you required to use these words to create a wallet seed?</strong></p>
</blockquote>

<p>A BIP39 compliant wallet will insist the words be in that list. There are different lists for different languages.</p>

<blockquote>
  <p><strong>What would happen if you chose random words not on the list, or instead a long continuous string?</strong></p>
</blockquote>

<p>If you try to use the wallet-recovery feature and type in words not on the list, your wallet would display some sort of error message.</p>

<p>When creating a wallet you generally do not get to choose the words yourself. People are very poor at choosing words randomly and this results in very weak seed phrases from a security point of view.</p>
 7272: <p>Technically, an HD wallet does not require a mnemonic. It simply requires entropy. The mnemonic system is designed to make human use easier, as it is far easier to write down a set of words, instead of trying to safeguard a long hex string.</p>

<p>The wordlists themselves are chosen to have specific properties, namely:</p>

<ol>
<li>The first four letters are unique for each word across the entire list, which makes noting it down easier, especially with compact backup devices like a cryptosteel.</li>
<li>The list is sorted alphabetically, which makes lookup easier</li>
<li>The list avoids words that are similar looking and sounding, which makes it easy to correct improperly noted down words.</li>
</ol>
 5273: <p>The bip39 algorithm is badly designed and that is why it is dependent on a specific wordlists. Each time you want to add a new language you have to update the BIP! </p>

<p>Electrum has it's own seed mnemonic format that does not depend on specific wordlists so it's definitely possible to create a format like that. </p>
