Because it can be memorized and works reasonably well:
unsigned long hash(unsigned char *str) { unsigned long hash = 5381; int c; while (c = *str++) hash = ((hash << 5) + hash) + c; /* hash * 33 + c */ return hash; }
All one needs to remember is a single line and a number. For years SuperFastHash had been my first choice but the Non-Cryptographic Hash Function Zoo analysis changed this.
[via]