If they are in good shape, and very specifically if the track fits the new rollers… If new rollers are too small or too large they will have issues traveling in the track.
Old rollers bearing blow out and fail over time, and generally over time there are minor changes in how big the rollers are, probably specifically to make the installation a bit more expensive with a new installation. The other issues with the old track are that it can get out of true (level, parallel, straight, etc… ) old track often should be at least re-adjusted, which can often take longer than installing new track (and will cost more due to labor hours) So it depends.
If you’re doing it yourself and have to squeeze the pennies… you CAN re-use track. It will take a bunch more effort to do it right (in researching correct rollers, measuring, bending, perhaps re-welding joints, cleaning, leveling… ) so yes, but it isn’t always the most time-efficient, and can, if done poorly, cost more than replacing the track due to failure on the door due to poor fit.
If the rollers are right for the track, and the previous door didn’t fail by coming out of the track (bending it on the way often) you can keep the old ones… installers would rather use the new track, as it eliminates all the potential issues, and allows you to have a warranty which if you re-use the track probably not.
The springs are wound up to assist in lifting the weight of the door.
If you add lots of insulation to your garage door, it will cause the door to be heavier and the springs had to be tightened up with more turns. Well, when the weight was removed, the door would not stay closed.
Countless people have been KILLED trying to work on springs on garage doors.
PAY FOR THE DAMN PROFESSIONAL GARAGE DOOR GUY, they have tools and they know, how to do this job safer then you could.
You can watch the guy the whole time and ask questions, you will think you could do the job, but you do not want to be that guy when the springs pop loose.
The springs are used to assist the garage door in lifting up the door. Without the springs the garage door opener would need to be stronger and the substructure holding it wound need to be seriously strengthened to support it while it is in use. With things like the springs and a lower-powered garage door opener it makes it cheaper and easier to own, maintain and install a garage door opener is almost any garage.
Because a garage door is very heavy. You would probably not be able to lift it without the help of the springs. The springs act somewhat like a counterweight, applying force to the garage door, which reduces the amount of force needed to open the door.
The springs also help to keep the garage door from crashing down with excessive speed and force, which is likely if there were no springs to apply force in the opposite direction.