While Tactical Marines, supported by Assault Marines or Devastator squads, will be your main strength, Tactical Marines are poor for map control. While Tactical Marines are great fighters, they are also extremely slow. Because of this, you'll have a hard time keep your control points from enemy squads with only your Tactical Marines. You'll basically be playing whack-a-mole, and you'll also be in a constant position of reaction.
That is where Scouts come in. Scouts are poor combat units in most cases, but they are you fastest unit, and when upgraded with Shotguns they can fend for themselves against basic infantry squads. You start with one Scout squad, and you should use it well. The Scout Squad should never be idle. It should always be moving from one point to another, constantly threatening enemy control points. This ensure that your army will not be constantly split trying to take strategic points.
Generally, you should keep your Scouts away from combat. But if you do get into combat, remember to use the tools they have. A Scout squad that is dedicated to capping points should be equipped with Shotguns, which are good against most infantry. The Shotgun upgrade will also give you the infiltrate ability, which allows you to hide your scouts from an overwhelming force. Infiltrate isn't just for hiding, however, because it provides a passive defense bonus when it is activated.
Get used to switching between your Scouts and your Tactical Marines. This can take some getting used too, because Tactical Marines and Scouts are such different units. But you'll have no choice against skilled opponents. The Orks, Eldar, and Tyranids will all be fielding more units and faster units then the bulk of your army, and without your scouts you will never be able to keep map control.