You will get some Avios extra (eventually).

There are also certain legal safeguards that apply for Holiday bookings, so for example if you are bumped off a service and get to your hotel a day late, normally BA won't pay consequential losses, but if it's a Holiday booking then the shortened hotel stay is covered by regulatory protection.

24 hour helpline (though admittedly the FT Dashboard gives the numbers for getting this on other bookings).

Ability to pay by installments.

The advice I (and many other regulars) give is to always take your booked flights. BA are clamping down in this area, but there are no reports of this happening on Holiday bookings yet. You can also do flight + hotel on a single booking but the price isn't too attractive usually.