What are they for?

To help you remember! =)

For a better answer:

Essentially they act as self customized reminders. Most people use the radicals in the characters as hints. I'm personally a little different though, because I create little stories for each character. for example 船= chuan2= boat. My mnemonic for this character is as follows: Above view of a ship, with a square floating down the river. Your imagination is the limit. =)

I HIGHLY encourage learners to use mnemonics, but be careful not to become to reliant on them.

Thank you kind sir.