When are bonds actually superior? I don't get it.

I understand what bonds are and what they do, but I don't get why they occupy such prominent positions in some portfolios, given how they behave. I looked through a bunch of bond ETFs, and they tend to fall in two buckets: straight line up (which is nice and low risk, but I can earn more interest on cash in the same currency as the bond's...), or jump around randomly, mostly achieving nothing. Interest on cash still beats these bonds comfortably.

I was especially looking at some of the past crashes and how USD, EUR, JPY, GBP bonds behaved then, and I found largely the same behavior. TIPS went DOWN for the 2008 crash, and only started performing well in 2010, well into recovery. Might as well buy the dip on stocks... The only good movement it had was Aug 2008, but market timing in hindsight is easy.

What am I missing? When are bonds actually superior?