"Alan Erskine" wrote in
:

...why do the SSME's have an ignition system? Why don't they just mix
a small amount of H2 and O2 in an ignition system?

The SSME has an internal ignition system, in both gas generators;
they're literally spark plugs. Necessary since a precisely controlled
startup sequence is required and just mixing the two propellants won't
result in ignition, unlike hypergolic propellants.

If you're referring to the external spark generators, that's only to
ignite venting hydrogen before it builds up to explosive levels, which
might happen if the engines are shut down on the pad.

The visible venting on the engines prior to ignition is oxygen; vented
hydrogen is piped away and burned off some distance from the pad
on a flare stack.

--Damon