I mostly do it for the autocompletion, really. By typing _pv_ it instantly eliminates everything but the private variables of the class. Having the autocompletion sorted alphabetically into sections just feels neater too, ya know?
The n for integers, f for floating points, p for pointers etc. is also reminder for me, on top of nice to have all of them grouped alphabetically by ac, because even if the IDE tells me the type; if all I want to make is a quick glance through a function it just helps
The amount of times I forget a variable name and go
"Hmm, now I know it's an integer and private so... _pv_n ah, there it is"
The I/C/B/SI layout helps me keep things organised in my mind more than anything else. Wanna limit the amount of times I have to open the class or my half-baked design plan file, so anything which acts as a quick reminder helps
I don't always make what I mark as an interface pure virtual though because.
Of course, both of us will have to abandon our ways if we get involved in team professional projects
They always have an agreed-upon notation system :p