While not one of the flashiest models in the Space Marine codex, I think the humble Scout Biker squad is one of the better value units in the book.
First, let’s look at their stats. For 4 points more than a normal Tactical Marine, a Scout Biker has:
-12″ move, with the option to turbo-boost, so MUCH better mobility than Tactical Marines. Additionally, they have Infiltrate and Scout moves, which gives them TONS more mobility options, both during deployment and afterwards.
-TLed BS3 Bolters, so actually more accurate (slightly) than a Tactical Marine with BS4 Bolters. Also, Scout Bikers can take up to 3 x Astartes Grenade Launchers per squad, which is a pretty solid supporting special weapons set.
-Durability-wise, Scout Bikers lack a 3+ save (4+ instead), but are T5 and can Jink for a 4++ cover save (making them better against AP3/2/1 weapons). Against Bolter fire, it would take 9 x Bolter shots to kill a Tactical Marine, while it would take 9 x Bolter shots to kill a Scout Biker, so basically the same toughness against small arms fire.
-Scout Bikers are scoring (like everything these days), but don’t have Objective Secured and don’t take up a mandatory Troops slot, so they have disadvantage there. That said, their superior mobility means they are much better at dashing around and picking up objectives in Maelstrom of War missions.
Now, one of the biggest things Scout Bikers have going for them is Locator Beacons, for only 2 MBs cost to upgrade. Combined with their Infiltrate, Scout move, and up to 24″ movement a turn, Scout Bikers can bring in units accurately from Deep Strike Reserve almost anywhere on the table. There are numerous applications for this with Imperium allies, whether it bringing in allied Terminators, Deep Striking Scions, Grey Knights of all kinds, Jump Troops, Land Speeders, etc. etc.
One of the most unique possibilities for using Scout Bikers is combining them with a unit that has Gates of Infinity from the Sanctic discipline… Draigo comes with this power automatically, so attaching him to a powerful squad like Centurions or Hammernators and then “bouncing” them around the board with the Scout Bikers “pulling” them in with their Homing Beacons for no scatter is quite a creative tactic. Even after the Scout Bikers have “pulled” in their selected units they are quite useful, providing extra shooting, tying up critical enemy units in close combat, or even glancing down an enemy tank with Krak Grenades and Melta Bombs (on the Sergeant).
Overall, Scout Bikers are quite a versatile, cheap, and useful unit that open up quite a few possibilities for a creative army commander.