Insulation and roof venting
In climates where you’re heating more than cooling during the year, yes, it’s advisable to add attic insulation, but: 1) you must maintain CONTINUOUS ventilation, and 2) you must maintain a minimum of 1” air space between the insulation and the roof structure. 

1) You want “cross ventilation”. Your soffit vents and ridge vents are doing that now. Just make sure EACH JOIST SPACE has a soffit vent AND a ridge vent after you install the attic insulation between the joists. 

2) Make sure there is a minimum of 1” air space between the insulation and the roof structure, especially at the soffit vents. Insulation tends to “fall down” and block the soffit vents, so they often use cardboard baffles to maintain a clear vented opening.