It gets (ab)used to store tons of semi structured data with a pretty good siloing and acl capability (including allowing Google 3rd party partners to work on issues). This data includes bugs but also tasks, workflows, approval processes etc. Buganizer is very api-first so many different tool frontends use it as their backend.
Generally every huge leap in bug numbers was associated with buganizer assimilating another tracker's bug database (from acquisitions and similar). For what it's worth no issue tracking system I am aware of has avoided buganizers huge gravity well. I expect this trend to continue.