I think, it's for the secure of your wallet.
Cos, if you just have 1 address and you store all of your Bitcoin in it. When someone hack your account, boom|You lose all of your Bitcoin.
But if you store the Bitcoin to every different address, then if someone hack 1 address you'll just lose small amount of the Bitcoin.
How could a person "hack my account". If they did that would they not have access to all 100 addresses?
Once you have transmitted some bitcoins from an address the private key behind that address becomes mathematically less secure. This is because some information is revealed on the public blockchain that would make it slightly easier for a hacker to guess your private key. Using today's computing power, this does not amount to any significant risk, but who knows what can happen with future technologies such as quantum computing.
So with 100 addresses each with different private keys this theoretical risk is mitigated.
In practice, having 100 addresses is only better for privacy reasons because it makes it a little harder for people to track your transactions.