main reason given by developers was, they need ai for conferences and events trackside, so people can for example hop into a quick game with 30odd ai at daytona at the iRacing booth during the daytona 24h. that's what was missing and that's why they do the ai.
as was said above, for the average player it will not make much sense, the base content in iRacing is fine but hardly breathtaking and buying/renting new stuff for about 12 euros one piece is hardly appetizing either. the iRacing guys - like me - pay the money to do online racing, fullstop.
just finished a great gt3 race at monza, lowest grid of seven, each grid consisting of 30 players, the amount of players is still unbeaten by a long long shot. even an ultra-slow chap like me gets gridded with chaps in a range of +1.5 to -2 secs off my time, so really close competition. finished the 40 min session with never a player making a stupid move on me but with a couple of nice, intense, fair fights. lowest grid is actually quite clean because most of us there know how dumb we are, lol. in the middle grids people overestimate their skills and - blam!