Tactical Marines are a solid ranged squad. They also have -40% received melee damage, and deal more melee damage than most ranged units. So, not a highly recommended move, but you always have the choice of temporarily ordering them to easily beat up most other factions' ranged squads (except Chaos Space Marines which are like Tacs, but Kraken Bolts also increases melee damage so they can get an edge over the CSM if needed). You should still avoid brawling enemy melee specialists on purpose, however.
More importantly, they have special weapon upgrades that are each good against different types of foes, and they have the unique ability to change their special weapon upgrade whenever they want, making them very flexible. However, you will have to pay for each one (no refund for the old discarded weapon upgrade), so use this ability sparingly (preferably only in emergencies).
Using the flame_pvp damage type that does a whopping 5x damage to building_light armor type, it can be used in early power farm attacks, slowing down the enemy's power economy and helping put you ahead despite the flamer's own 20 power cost. The power farm-raiding can also help build up Zeal for summoning Terminators later.
Due to its area-effect splash damage, it is also an effective counter against squads consisting of a large number of weak models ("hordes"), for example Termagants or Guardsmen, because it will inflict its full damage to every weak member of the enemy squad within the zone.
A very specific anti-Heavy Infantry upgrade. Against normal Infantry enemies, It only has a very slight boost above the damage of the default bolter (about 28% more), making its high cost inefficient. However, against Heavy Infantry, the plasma gun cuts through the enemy like butter, doing over 4x more damage than the bolter.
Very expensive upgrade that is usually only used in emergencies when anti-vehicle firepower is suddenly needed. Only about one-third the damage of a Devastator lascannon and shorter range. However, after firing each missile, the squad can displace and move during the missile launcher's long 6-second reload time, providing more tactical flexibility compared to weapons requiring a set-up time.