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Because BFD is not tied to any particular routing protocol, it can be used as a generic and consistent failure detection mechanism for EIGRP, IS-IS, and OSPF.
"Generic", yes, "consistent", maybe, maybe not. BFD isn't forced on the IGPs (or FHRPs). BFD is something IGPs and/or FHRPs can take advantage of. BTW, keep in mind that later IGP and/or FHRP protocols often support their own fast failure detection w/o BFD.
Because some parts of BFD can be distributed to the data plane, it can be less CPU-intensive than the reduced EIGRP, IS-IS, and OSPF timers, which exist wholly at the control plane.
Well, in theory, no reason why IGP and/or FHRP fast detection implementations also couldn't be implemented in hardware like BFD. (Although "sharing" a timer, between multiple "users" would likely be much more difficult without BFD.)
Conceptually, BFD is to fast failure detection like TCP is to data transport. I.e. it addresses a problem that other multiple services have.
