Skecth has too many good things but I will only list the ones that are the most important to me. First and by far, what I like best about sketch is its solidity to work. It is - within UI design programs - the one that works best, restarts not very often and can deal very well with very heavy files. Being specially built for the MacOS platform, it makes it very stable. I have worked with other new design programs that are electron based and are very slow.
Particularly in my point of view, the fluidity in the use of a program, is very important and Sketch has it.
Another feature that I really like about sketching is the amount of online resources that exist. Being a standard in the industry, there is a lot of community that creates shareable resources so - for example - if I need an example of a dashboard design, I find it very easily.
Finally, I must say, I highly recommend Sketch as a professional tool for UI design.