user88669: A lot of bitcoin exchanges require some minimum amount of confirmations before they consider money deposited. 
Is there any evidence that this actually does anything? Has someone ever before manipulated a transaction on the bitcoin ledger because the recipient didn't wait for 10+ confirmations before accepting that the bitcoin has been exchanged successfully. 
Or is this just to stop a theoretical attack from happening?

user85140: What you are referring to is likely a preventative measure for the possibility of a double-spend attack. Due to the nature of the Proof-of-Work algorithm in Bitcoin, the likelihood of such event on a transaction with 6+ confirmations is very low due to probability and the cost implications of re-calculating X amount of blocks.

Is there any evidence that this actually does anything?

Suppose you have a large transaction that is very attractive to an attacker or someone looking to perform a double-spend attack. Once the transaction is included into its first block, it is considered to have 1 confirmation. Each block built on top of this chain adds to the number of confirmations of the transaction. This makes it increasingly difficult for a double-spend attack because the attacker would have to complete the same amount of computation (proof-of-work) to become the larger chain. In our example, 6 confirmations would require the attacker to mine 7 valid blocks to become large enough to be considered the valid chain. For example, having 20% of the total hash power of the network with 6 confirmations would yield a 1.42% probability of a successful attack. This number becomes exponentially smaller as the number of confirmations increases.

is this just to stop a theoretical attack from happening?

One important aspect of Bitcoin is that nodes will always favor the blockchain with the most amount of proof-of-work (longer chain), this is critical to preventing double-spending.

Has someone ever before manipulated a transaction on the bitcoin ledger

This is a slightly controversial question, and I would recommend reading this thread for various attack specifics.
The Bitcoin Wiki also has several charts and calculators that can be used to determine theoretical attack probabilities. 

88669: <p>A lot of bitcoin exchanges require some minimum amount of confirmations before they consider money deposited. </p>

<p>Is there any evidence that this actually does anything? Has someone ever before manipulated a transaction on the bitcoin ledger because the recipient didn't wait for 10+ confirmations before accepting that the bitcoin has been exchanged successfully. </p>

<p>Or is this just to stop a theoretical attack from happening?</p>
 85140: <p>What you are referring to is likely a preventative measure for the possibility of a double-spend attack. Due to the nature of the Proof-of-Work algorithm in Bitcoin, the likelihood of such event on a transaction with 6+ confirmations is very low due to probability and the cost implications of re-calculating X amount of blocks.</p>

<blockquote>
  <p>Is there any evidence that this actually does anything?</p>
</blockquote>

<p>Suppose you have a large transaction that is very attractive to an attacker or someone looking to perform a double-spend attack. Once the transaction is included into its first block, it is considered to have 1 <a href="https://en.bitcoin.it/wiki/Confirmation" rel="nofollow noreferrer">confirmation</a>. Each block built on top of this chain adds to the number of confirmations of the transaction. This makes it increasingly difficult for a double-spend attack because the attacker would have to complete the same amount of computation (proof-of-work) to become the larger chain. In our example, 6 confirmations would require the attacker to mine 7 valid blocks to become large enough to be considered the valid chain. For example, having 20% of the total hash power of the network with 6 confirmations would yield a <a href="https://people.xiph.org/~greg/attack_success.html" rel="nofollow noreferrer">1.42% probability</a> of a successful attack. This number becomes exponentially smaller as the number of confirmations increases.</p>

<blockquote>
  <p>is this just to stop a theoretical attack from happening?</p>
</blockquote>

<p>One important aspect of Bitcoin is that nodes will always favor the blockchain with the most amount of proof-of-work (longer chain), this is critical to preventing double-spending.</p>

<blockquote>
  <p>Has someone ever before manipulated a transaction on the bitcoin ledger</p>
</blockquote>

<p>This is a slightly controversial question, and I would recommend reading <a href="https://bitcoin.stackexchange.com/questions/722/has-there-ever-been-a-successful-double-spend-attack-on-the-bitcoin-network-if">this thread</a> for various attack specifics.</p>

<p>The <a href="https://en.bitcoin.it/wiki/Confirmation" rel="nofollow noreferrer">Bitcoin Wiki</a> also has several charts and calculators that can be used to determine theoretical attack probabilities. </p>
