When was the last time you checked your tyre pressure? Don't remember? Or maybe it was done the last time you had an oil change (or at least you hope it was). Most people don't care about tire pressure because they don't notice anything wrong with their vehicle.
When a tyre is underinflated, most of the car's weight is concentrated on the tread that is located just under the sidewalls, rather than being spread out evenly across the full width of the tyre. This means that as the tyre rolls, the sidewall gets continually flexed (squished, if you will) and heats up. This affects both performance and safety. In addition to degrading the handling of the vehicle, a tyre that is considerably low on air can blow out due to the stress from the heat buildup and the constant flexing of the sidewall.
Check your tyre pressure at least once a month, perhaps on the first of the month to make it easy to remember. And while you're at it, give the tyre a once-over and look for uneven wear patterns as well as cut or bulging sidewalls. Irregular wear could signal the need for an alignment or replacement of worn suspension components. A deep cut or a bubble in the sidewall indicates a potential weak spot that could lead to a blowout.
And you don't want to overinflate tyres either, as that won't allow the desired full tread contact (due to the car riding chiefly on just the centre of the tread) and will make the ride stiff. Check in the manual for the correct type pressure for front and rear tyres.