TIFFs can have Layers just like psds so thats one advantage.
They can be smaller files sizes than equivalent psds...thats an advantage.
Photoshop can export a psd (work project) as TIFF
They can be exported in a SEQUENTIAL File system
From your description, it sounds like he was rendering some precomps or layers as an image sequence rather than leaving the comp live.
This has the benefit of AE not having to render that precomp or layer. If you have a very intense particle effect with hundreds of thousands of particles, and you know it's not going to change, it can help to prerender it so that the rest of your work can continue without having to re-render those hundreds of thousands of particles every time.
Does that answer what you were asking?
A tiff sequence is a universal format readable by any nonlinear editor or compositing program. It is a lossless format and it is available at high bit depth. For clinical work it is a good choice. If you need to change ten frames is in the middle of a sequence also a good choice because you can just fix those 10 frames.
I think so, thank you. So, it's basically a different way to render a precomp, to cut down on the processing needed, so to speak? And yes, his animation was VERY busy, with lots of elements.
I'm guessing the reason I've never done anything with it, is my aesthetic style is more clean and simple, with much fewer elements, so the need for this process has never really popped up in my own work.
Thanks for the help in explaining. That at least helps me get into his mindset for his decisions.