Why does this regional jet have its air brakes wide open before touchdown?
(Source) F-15 landing with airbrake extended.
(Source) Airbrakes on the Fokker 70.
1. It allows steeper approaches
From Wikipedia:

The engines [on the Avro] lack thrust reversers due to their perceived reduced effectiveness in anticipated conditions. Instead, the plane features large airbrake with two petals below the tail rudder at the rear of the fuselage, which has the advantage of being usable during flight and allowing for steep descent rates if required.

... also...

Airbrakes on the Fokker 70's tail section – similar to that found on the BAe 146 / Avro – allows it to conform with the 5.5° glide slope at London City Airport.

2. It allows better speed control
Some aircraft are overpowered with high power-to-weight ratio. The thrust lever range alone won't be enough to hold the approach speed.
The thrust-to-weight ratio for the RJ100 is 0.28:1, compared to 0.16:1 for the comparable Boeing 717. It's even higher for the smaller RJ's.
Low power and the plane loses speed, some power and the plane gains speed. So, more power is used in conjunction with airbrakes, such as the F-15 shown above.

Once on the ground, they help slow down the plane, same as spoilers.
(Source) Spoilers.
Spoilers have the added benefit of spoiling the lift–transferring more weight to the tires for increased braking capability.

Related: Why do some aircraft (e.g. Avro RJ85) have rear-mounted air brakes?