Do i have to use the codes i won ?

Yes
From FAQ:
You should only enter to win giveaways for yourself, and if you win a game, it must be activated and redeemed to the Steam account used during registration. [...]

Yes

?

Well, you don't have to, but you'd be violating SG policy if you don't and that is grounds for suspension.

Won gifts should be activated to the Steam account used during registration, and they should not be regifted, traded, or sold.
https://www.steamgifts.com/about/guidelines

That's why the site exists.

You should only enter to win giveaways for yourself, and if you win a game, it must be activated and redeemed to the Steam account used during registration. If a friend is interesting in joining giveaways, they'll need to register for their own account.-FAQ

That depends how long you intend to stay.

You also need to read the rules anyway.

FAQ Guidelines under "Giveaways" > "Winning" >

Won gifts should be activated to the Steam account used during registration, and they should not be regifted, traded, or sold.

Terms of Service under "If You enter a Giveaway" >

any Gift you receive is for personal use, and must be activated, or redeemed by the Steam ID associated with your SteamGifts.com account. The Gift should not be used for other purposes, including by not limited to, trading, selling, or regifting.

i know ppl alrdy said the "FAQs" part, but few people know it is actually even more clear and firm (with the must be activated) in the ToS

You don't exactly "have" to use the games you win, nobody is keeping you at gun point to force you to activate them.
BUT you MUST activate them on the LINKED Steam account, if you don't want to have a bad (and possibly short) time on this site.
Then, once the game is in your Steam library, you're free to do anything with it: playing it, idling it for cards, ignoring it... the only thing you CANNOT do is removing it.

By the way, why are you even asking?
If it's because you won a crappy game, you can ignore the key/gift, write a comment in the giveaway to let the creator know you rejected the gift, and that you agree to do a re-roll.
If it's because you won a DLC you already own, you should let the creator know, and ask them to request a re-roll.
If you won a region-locked game that was not labelled as such, things might get trickier...