Powdered rosin is useful because when new hair has no rosin on it at all the rosin you are trying to apply doesn't have anything to stick to, so it takes a while to get your rosin on the bow. If you are using a cake of rosin, it will often "skate" across the hair and eventually find spots that starts to grab rosin while the rest doesn't, and it takes a while to fill in the gaps and get a consistent amount of rosin from one end to the other. It takes a while, and if you're starting with a fresh cake of rosin that has a glossy new finish on it without roughing the surface, very few violinists want to go through it themselves and often request powder. The powder is just regular rosin crushed up and then brushed onto the hair. It gives your rosin something to grab onto, and can be distributed evenly to prevent the "spots" issue.
With the really hard and powdery upper string rosin you can blend in the spots with too much rosin pretty easily by playing a few long full bow strokes from one end to the other, and letting the string/hair even things out on it's own. It's also harder to over rosin with hard rosin, because it goes on slower.
With bass rosin, there are two big ideas:
Bass rosin(s) are very different from upper string rosins and often each other, so some bassists want to exclusively use the one rosin they like on their hair. Some rosins work well together, some don't, and it doesn't really take that long to get something as sticky as pops on fresh hair anyway, so leave it alone.
Bass rosin(s) are very soft, so it's easy for them to clump/blob and you to over rosin. On new hair one spot grabbing rosin and the rest of the hair not can mean way too much rosin in one part but not the rest, and spreading it around by playing it in can be tricky, as it tends to stay put instead of disperse the way the harder violin rosin does. Just like the upper strings powder can help evenly disperse your own rosin which reduces the chance of over rosined random spots and other places where the new hair refuses to take the rosin.
All of that should be pretty much worked out in your first practice session after a rehair anyway. Depending on how long it has been and if you had other work done when the rehair was happening it might take a while for you to remember what your bow feels like when it's healthy again, but it shouldn't be some week long affair to get it going. Usually when that's the case there are other variables like you've changed rosins or strings or had work done to your instrument or some combination of all the things and you are adapting to all of them together and that's a longer process.
Don't get it rehaired and go directly from the shop on stage for a performance without any time to warm up and think "my usual 2 swipes of pops should do it" but you don't have to plan a month in advance to appease the rosin gods.
