Yes
Source: used flaps only once

Thanks, I'll try messing around with them next project.

Short answer: yes, flaps change flight properties and when done correctly they increase lift.

Longer answer: you have to be careful how it's done. Main wing physics get a bit weird in SP so if you have very long (not wide, but long - nose to tail) wings sometimes the control surfaces work BACKWARDS. Most successful flaps use xml modding, rotors, or both.

Also, some have noted that flaps increase drag. This is true, because flaps increase lift and when it comes to wings lift IS drag. Basically, wings use carefully shaped drag to convert wind speed into lift, some wings are more efficient than others, using less drag to create more lift but almost all wings are only effective between the specific speeds and altitudes they were designed for. That's why flaps exist, they allow the wing to change shape and provide lift at lower than normal speeds.

Yes, when i use my fighter (without flaps) i have to look about 15° upwards when i want to stay at about the same height when im near stall speed. When using flaps, i only need about 5°

Flaps made with a wing panel and rotators or flaps with a progranmed control surface set as input with vtol thru xml or overload do help create more 'lift' for landing. Arguably its basically an extra set of trim physics. So yes they do also increase your drag points. Alot of users create main wing flaps mostly connected to the Vtol slider. Which do aod a craft stay level while in a lower speed approach for landing.

Flaps are, at times cosmetic in-game, but there are many who make flaps actually work. They do provide lift but drag as well.

I'm pretty sure they create drag, but I don't know if they create more lift as well. I've seen a lot of builds use them though, so I would assume flaps do work in-game.