The improvement isn't amazingly better-but I had SLI for two 8800 gtx's back in the day, and that powered me through another generation of games (and PC build).
From what I remember, the big benefits is dealing with super high resolutions.
If it doesn't cost you much more, it's worth the improvement imo.
Did a few SLIs on my water-cooled rig some time ago, in my view its not worth the hassle. It works only for certain games, there are certain limitations and its not as foolproof as just plugging one card. Most of the SLIs out there are either miners (waster of time) or guys who want to show off benchmarking. Having said that if you've gotten one for free and assuming your PS is powerful enough the only investment you should do is the SLI cable. In that case, give it a go. But if you have to put more money into it, rather consider a single, more powerful card
Make sure to check which games you play and if they benefit from SLI. Most games are poorly optimized for SLI and require a bit of handy tweaking to benefit. Since you already have it go for it and see. Witcher 3 is a fantastic game but there seem to be quite a few issues with SLI if you google it.
My understanding is that if you use SLI the two cards will act as one graphics card which is ideal if you want more performance for gaming. I am not aware of any specific benefit of two cards without SLI for gaming, but maybe anyone better informed can post. Read somewhere that potentially you could assign physx to one card for some benefit but the game need to support it.
Using two cards without SLI can also be useful if you want to assign specific tasks e.g. one card for monitor, one for GPU tasks or specific software/application etc. if you have a user case is this area it could be worth exploring.