Other software Substance painter/Zbrush ect

Hello!

I see alot of peoples work from a mixture of blender/zbrush/substance and photoshop ect.

I get photos shop, tweeking of a post production render makes sense but I am interested to know why you might use these other peices of software when blender has sculpting and rendering capabilities.  Is it just what your used to using or do they offer something that blender doesnt?

So I'd like to know what software people use in conjunction with blender and why?

Should I perhaps be using another peice of software for best results(so i cam decide if i should invest in it or)?

Thanks