The intention was to fit the GW main line with it, and then the ECML.
But the GW plan was canned, except that the Heathrow branch was fitted.
That means Heathrow Express needs it, and also Crossrail when it reaches Heathrow.
HEx decided to use GWR 387s instead of refitting their 332 fleet, hence the project to fit the 387s.
It means GW ATP can be retired on the branch.
It’s a bit more complicated than that.
The Heathrow branch was only fitted with ATP, and not TPWS.
The 332s are maintained at Old Oak Common in a depot that has to be demolished for HS2.
GWR have spare 387s with the ‘pausing’ of Oxford electrification;
387s are not fitted with ATP
345s for Elizabeth Line are not fitted with ATP, but do have ETCS (as almost all new trains have now).
To get the Elizabeth line to Heathrow, it was easier to fit that with ETCS than fit the whole 345 fleet with ATP.
It was much cheaper to use the spare 387s on HEx, adding ETCS to use on the Heathrow branch (which the 387s will need at some point anyway), than to find a location for, and build, a new depot for the 332 s.
Phew.