Why does Signal repeatedly hash the secure passphrase?
Cryptographic hashes are designed to be fast and collision resistant. It turns out that when hashing passwords, it is more secure to have a slow hash function. One way to make a fast hash function slow is to iterate it. Like is done here.

Think about it this way. If an attacker is able to compute a million hash calculations in a second, if you only ran your password through the hash function once, they would be able to attempt a million guesses each second. On the other hand, if you iterate the hash function 250,000 times, now the attacker can only make 4 guesses a second. That is a pretty substantial reduction and increases the amount of work the attacker has to do. If users were really good at choosing passwords, this might not matter, but the fact of the matter is that users are not good at choosing good passwords. Making the hash function slow helps in that regard.

In the case of Signal, it looks like encrypted backups is a beta-feature. Looking at one screenshot, it appears that the backup key is not user chosen. It is likely (haven't dug through the code) a 30 digit random number. That means it has about 100 bits of entropy at best. My guess is that the designers still wanted a slow hash function for the reasons listed above, even though the passphrase was not user chosen. Making it slow is a good trade-off for having less entropy. They could have made the random number longer to get more entropy (256 bits maybe), but that would have made the backup key that the user has to store much longer.