user140388: There are Bitcoin wallets like "Sparrow" which use a central service for maintaining the Blockchain, so I don't have to download 400GB to start using Bitcoin.
Now, why should I use Bitcoin Core (full node software) instead of a Bitcoin wallet software like "Sparrow" which is using a centralized server, which the software is able to contact to ask for my balance?
There is also other software like "Electrum" which only differs in the way that Electrum uses several servers and not only one, and it's running simplified verification.

user134811: Because running a full node grants you security and privacy.
The former because you do full verification of the chain, and the latter because you don't hand over all of your addresses to some stranger.

user13866: According to Sparrow

The following sections describe 3 common stages in a Bitcoiners journey.
[...]
consider that you are sharing your balance and location via your IP address.
If this thought makes you uncomfortable, it’s time to consider running a Bitcoin Core node

According to the creator of Bitcoin:

A purely peer-to-peer version of electronic cash would allow online
payments to be sent directly from one party to another without going through a
financial institution. Digital signatures provide part of the solution, but the main
benefits are lost if a trusted third party is still required to prevent double-spending.


140388: <p>There are Bitcoin wallets like &quot;Sparrow&quot; which use a central service for maintaining the Blockchain, so I don't have to download 400GB to start using Bitcoin.</p>
<p>Now, why should I use Bitcoin Core (full node software) instead of a Bitcoin wallet software like &quot;Sparrow&quot; which is using a centralized server, which the software is able to contact to ask for my balance?</p>
<p>There is also other software like &quot;Electrum&quot; which only differs in the way that Electrum uses several servers and not only one, and it's running simplified verification.</p>
 134811: <p>Because running a full node grants you security and privacy.</p>
<p>The former because you do full verification of the chain, and the latter because you don't hand over all of your addresses to some stranger.</p>
 13866: <p>According to <a href="https://sparrowwallet.com/docs/best-practices.html#the-intermediate-stage" rel="nofollow noreferrer">Sparrow</a></p>
<blockquote>
<p>The following sections describe 3 common stages in a Bitcoiners journey.<br />
[...]<br />
consider that you are sharing your balance and location via your IP address.</p>
<p>If this thought makes you uncomfortable, it’s time to consider running a Bitcoin Core node</p>
</blockquote>
<p>According to the creator of Bitcoin:</p>
<blockquote>
<p>A purely peer-to-peer version of electronic cash would allow online
payments to be sent directly from one party to another without going through a
financial institution. Digital signatures provide part of the solution, but the main
benefits are lost if a trusted third party is still required to prevent double-spending.</p>
</blockquote>
