I remember my first Dash-8 ground school where the instructor pointed out that the Dash-8 was designed in the U.K (which basically means it's over-engineered and over-complicated) and built in Canada (where marijuana is legal).
Many of it's systems (e.g. DC electrical system, Cabin temperature control, and hydraulics) bear testament to this.
Some systems improved with the Q400, some (like Cabin pressurisation) went backwards. But overall there was an improvement.
To counter such improvement, Bombardier introduced the "Flight-Taxi" switch on the Q400 to control the ground spoilers.
In "Flight" mode (and in flight) the spoilers are depowered, and deploy as required to assist the ailerons in roll control. This is known as Flight mode. On landing they deploy fully, which acts as a lift dump and gives up to a 100m shorter landing roll. This is known as Ground mode.
In "Flight" mode, and on the ground the spoilers are fully deployed unless applying power for take-off.
If the switch is set to "Taxi" the actuators are de-powered and the spoilers retract. This is done so that the aircraft doesn't have it's spoilers deployed for movement on the tarmac and while taxiing.
Part of the line-up drills is to set the switch to "Flight", which deploys the ground spoilers. When applying power for take-off (power levers are greater than 12 degrees forward of flight idle) the spoilers retract.
One of the pilot calls during take-off is "spoilers in", or some such. If the spoilers don't retract then the take-off should be aborted.
However, if the pilot forgets to set the switch to "Flight", the switch will automatically go to the flight position on application of power (you hear a 'click' as the switch moves).
There is a config warning on the Q400, along with a switch that you can press to test config upon initial taxi. Ironically, pressing the switch will give you a config warning if the "Flight/Taxi" switch is selected to "Flight".
Clear so far?
As to why you the aircraft you see has it's spoilers extended on taxi, I can offer two suggestions:
1. The pilot forgot to place the switch to "Taxi" after landing (happens sometimes); or
2. The airline FCOM directs ground operations to taxi with the switch set to "Flight".
Unlike Boeing and Airbus (who control their FCOM and QRH, etc, very strictly) Bombardier allow individual operators to have significant control over how they operate their Dash-8's. What this means is that different operators may have significant differences in their checklists and call-outs.
Hope this helps.