[semi-extrinsic]

ELI5 (distilled from wikipedia page linked by GP, forgive me any errors):

Web servers use hash tables for storing per-request data. If an attacker knows the hash function (say, SHA1), they can create a few hundred requests that yield the same hash, giving hundreds of hash collisions and creating a Denial-of-Service attack with the same effect as millions of ordinary requests. It's a form of DoS amplification.

A keyed hash function fixes this by keeping part of the hash algorithm (the key) secret. You can turn e.g. SHA1 into a keyed hash function by e.g. HMAC, but that's computationally expensive. SipHash, being a "natively keyed" hash function, is much faster.

[daveguy]

Except if the hash is SHA1 (or any other non-compromised cryptographic hash) they would have a very difficult time creating requests that yield the same hash, that aren't the same request, right?

[semi-extrinsic]

I'm not sure, but I think if you're just interested in creating N collisions, as opposed to finding something that collides with given plaintext X, that the birthday paradox gives you a huge performance increase and makes it feasible. Also, I think many still use MD5 in applications where SipHash is intended (at least the Linux kernel does).