Explain that. The first part. I get the second part.
I would imagine that the FBI would have a place for a full size, compact, and sub-compact pistol. And would occasionally have use for a suppressed pistol.
So one basic pistol platform can do all of those. Your training session is one session, on multiple configurations of one pistol. Holster inventory is greatly simplified. There will be one training and service manual, rather than 3-4. The modularity allows the end user to fix any issue, no need for an armorer, just drop in a new trigger module or slide, send the old one either to the armorer, or back to Sig.
At pretty much every FBI office, you'll be able to convert your pistol for a new role, and they'll have gun and leather on hand for you as well as spare magazines regardless of the configuration. Weapon lights will now fit all models of pistols.
Basically any role that you would have for an FBI agent, can be filled with one pistol; that really simplifies things for an agency the size of the FBI.
You come into the office and find yourself doing under-cover work, or assigned to the SWAT team, swap top halves and magazines to fit the two roles; you're done.
For finding something to fit a wide variety of hands. There are 3 frame sizes, and then within each of those frame sizes, there are 3 backstrap sizes per frame.
For the armorers, they train on one pistol. Right now they have to be trained for AT LEAST 6 different handguns currently (both pistols and revolvers). Or maybe they just skip having an armorer altogether, and just let the end user swap uppers & lowers to fix any problem the may be having, then send the defective units back to Sig.