Winglets can have several effects that make them worth considering on some designs.

They increase vertical area. If the plane has a swept wing or a fairly far forward CG, this can increase stability. If the plane's CG is pretty far back, and/or the wing has a straight LE, then the wingtips aren't very far back, and this effect is lessened, but still non-0. This, of course, varies with the shape of the winglet. The more area closer to the wing TE and the less area towards the LE increases the stability effects. On most conventional planes, the stability effects are pretty small, but on flying wings, it tends to be a larger effect

They also affect the wing tip vortex. This can do a couple of things. One is that it can reduce or eliminate aileron flutter. I had winglets on my prior B plane for just this reason, that plane had a really bad aileron flutter problem with out the winglets, but with them, the flutter went away totally. Messing with the vortex can also delay the tip stall by creating smoother air at the wing tips. The result can be a tighter turn.

The downside to winglets is an increase in drag and weight and hassle. I'm not using them on my current B plane because I don't think the plane needs them, and the hassle makes it not worth while right now. I might play with some later on to see if there is an increase in turning, but I suspect on my current deisgn the drop in speed will be larger than the increase in turning ability.

Winglet shape and size is important as well, depending on what you're looking for. For better turning, I found having about 2" above the top of the wing and 1" below the bottom, at the wing TE, with about half that at the wing high point seemed to work well.

Winglets will sometimes snag a streamer that would have otherwise slid off the wing tip. This is espeically noticeable with forestry tape. A cut here or there is good, but having a streamer stuck right at the wingtip can be a real hassle.

And winglets will add a bit of strength to the wingtip, so the wing holds it's shape better in a glancing blow mid-air or dirtnap where the plane hits wing-tip first, such as a cartwheel. On the other hand, having winglet get torn off is a hassle, and sometimes increases damage to the wing, depending.