I guess this must be a seriously inane question, but still I can’t help asking it.
Why can’t I do a provisional cast on by simply casting on with a different yarn and then starting the to-be-used yarn and then unpicking the cast-on row when necessary?
Technically, you could. However, it’s way easier to rip out a crochet cast-on than to pick out a whole row of actually knitting!
The cast on end doesn’t pop out like a regular knit row. You could knit a few rows, then cut it off and pick out the extra, but the crochet method is easier in the long run.
That’s actually the provisional cast on technique that’s described in the book “Weekend Knitting”. It’s what I usually do if I have to do a provisional because the other methods just do NOT work for me. You don’t have to unpick the cast on row though. Just use a contrasting yarn that’s kinda slippery, like some kind of cotton, and do a few rows with that before switching to your chosen yarn for the project, then when you need to undo it, just take a fine pointed scissors and cut the constrasting row of stitches just below where your regular yarn starts. Then you’ll just have to pick away a few loose ends and carefully put the stitches back on your needle.