Not sure why you need cell service for navigation? If you are using a cell phone to navigate with, don't you download the maps you need before you take off on your trip?
You are correct between downloading maps and the devices internal GPS, cell signal is not necessary and never count on it solely for navigation. The OP's question is a bit unclear and weirdly formatted, but I think I get the gist. Having traveled with folks on several occasions who had cell boosters, situations often come up where it's darned handy so I'd still consider it a useful tool.
Cell signal is great for quick research of an area if your plans change on the fly or something unexpected comes up. Or you just plain made a mistake, forgot to download something 5 minutes online can fix. You don't have to go quite as far out of your way with a booster, so long as you can get the booster a clear connection via the more appropriately positioned antenna, you can actually have a conversation or look up information instead of driving another 5-10 miles hunting the tower. Perch yourself on top a hill and odds get even better.
So to be clear, as stated it has to have something to boost, so it's not going to give you signal where there is none. However, the antenna in a cell phone is awful for range, sitting inside a vehicle is awful for range, so it could get you signal your device itself can't pick up from the cab, and it's going to be better than getting out, climbing on the roof, and slowly rotating your body to find the angle where your head doesn't block that one bar you might pull in.
I've considered getting one but haven't reached a place where I think it's worth it to me for the cost. Don't expect miracles, but it can turn a signal the device can't pull in or detect by itself into a usable connection.
