No experience of creating them but as an occasional user I would say that I have generally needed one after the purchase rather than before.
That's an interesting observation. I realize this can apply to a huge range of stuff, but do you often do enough pre-purchase research to encounter such videos before you buy?
Thinking out loud... keyword research I've done suggests that potential customers very often search for solutions to problems... so how-to pre-purchase videos should probably tend to be less product specific and more generically task-oriented.
Post-purchase videos, on the other hand, might need to target the product name (and perhaps the task performed with the product) specifically, since I'm assuming that the searcher problems now involve product support.
In terms of video production... if you do decide to produce both types of videos, you should consider filming/taping them in the same shooting sessions, since it's likely that their image and production requirements would be similar.
It's the folks that show real passion that seem to succeed at it. If I smell a sales pitch I avoid.
I'm thinking that some tutorials could be informative and soft-sell enough that they would suffice for both audiences. IMO, successful video needs to have what I call a strong motivational dimension. "Real passion" is a very helpful way of describing what makes that work. It can be subtle, but it needs to be genuine.
There are all sorts of ways of shaping a video to enhance this. Most web videos don't think nearly enough about the emotional aspects of presentation... even in tutorials.