What are winglets for?

What winglets for? I can chose if an airplane can have winglets or not?

Winglets can lower the fuel flow (and therefore costs) on operating an aircraft. Only certain aircraft can have winglets, some you have an immediate option (737NG), some require winglets (A330), and some only have them available as a retrofit - as in, they aren't immediately available and can only be installed some time later/after its production (757)

Do winglets have some negative effect?

You gotta pay money

Aside from costs, not really. It's a little like deciding between buying a cheap car and a more expensive car that has better fuel economy (although the percentage difference in cost isn't nearly as much)

To make your airplanes look cooler.

Lower fuel costs vital for lcc carriers

Ok thanks

You gotta pay money

If you order them on new planes they're free.

(I suppose that's not very realistic, IRW ordering with factory winglets should probably be cheaper than retrofit but more expensive than no winglets, but to cut long story short in AE you get it for free)

And bonus is that they make a plane look awesome, apart from the 737max winglets, eugh