Basically what the title says.
When I was starting out and reading blog articles / watching videos, most of them seemed to make out like rye flour was optional to starting (and maintaining) a starter.
It’s not optional. Or at least, it wasn’t for me.
For two weeks, I tried to get my starter going using plain white flour. There was some activity, but not enough to bake anything good with.
One single 50g rye feeding was all it took to supercharge the starter. It literally tripled after that feeding. From then on, I’ve been feeding my starter a rye/white flour mix, and it’s as strong and healthy as ever.
So if you’re starting a new starter, or struggling with a weak one, go out and buy some rye flour. It’s well worth it, and your starter will love it.