I can only speak from my own experience which is in the last couple of months.
When we asked to book a second viewing the EA asked me to bring along our AIP.
The online one I did was for the top end of our budget, the affordability check we did with the IFA meant I knew we'd get AIP at that level.
When we made our offer I then had to forward the AIP I had got online & our solicitors details to the EA before the property was marked as SSTC.
We then went back to our IFA who did the full mortgage application with the lender we wanted to proceed with (different to the online one I got) and that was that, the AIP is simply the first stage of the application so your broker will do the AIP to check there's no issue then continue straight into the full application.
The one I did online contacted me about 4 weeks later & asked if I still wanted to proceed with the application, I says no thanks, that was that!
Obviously as I said, that's only my own experience, some EAs I spoke to wouldn't allow us to view properties without first forwarding an AIP, telling them we had it in hand via a broker/IFA wasn't enough.
For the sake of ten mins online it made everything simpler & quicker.
The AIP is literally an affordability check backed up with a cursory credit check - the real work is the actual mortgage application after you've had an offer accepted...