It is lightweight compared to photoshop and i do personally like the pen brush pressure of Paint tool sai compared to PS. The brush pressure is much natural to me without tinkering lots of of its settings. The simple UI of it will make the work faster like you can see all the settings you need on the side and lastly you can intergrate PSD files that you have drawn and edit in photoshop.

I like everything about PaintTool SAI. It doesn't use nearly as much GPU as Photoshop does (this means you can see the brushstrokes as soon as you press down your tablet pen, without any delay) and the brushes are just much easier to use than photoshop brushes. To this day I still don't know how to blend colours in Photoshop, but it is so much easier in PaintTool SAI. The user interface is very friendly and supports PSD files as well.

It is a very simple tool available on every Windows computer that can do basic tasks like cropping, combining, re-formatting images