The IRS Criminal Investigation Division deals with things like search warrants and arrests, so being armed makes sense. I don't think the bill calls for an additional 87,000 agents in the Criminal Investigation Division.
You know who else has their own law enforcement? BNSF. They are armed and have federal police powers. The likelihood of anyone on this board encountering a BNSF cop in the wild is infinitesimally small, unless you're a shitbag doing something very nefarious. Same with the IRS CI agents. Unless you're part of something like organized crime or a drug cartel, your chances of encountering an armed IRS agent is infinitesimally small.