However the best way to use it is to actually balance out the element of " Do i really need it" and " What actual unit i can modify to do the job better".
What " Do i really need it" means is like how does buying the item contribute to the army e.g Does buying an extra storm bolter for Space Marine Razorback helps? If it staying back far from enemy it aint doing much shooting. But one can argue that it helps when enemy are approaching the Razorback or improve Razorback suvivalbility as extra weapon can survive more weapon destroy results. Which lead to the next element which is "What actual unit i can modify to do the job" If the Razorback is constantly under attack maybe your assault unit isn't diverting enough attention or doing enough damage.
Let go though some of the common option that Vanila Space Marine codex you can buy for your vehicles.
Generally Rhino, Drop Pod and Vindicator have this by default and may opt to purchase an additional stormbolter. It great as it pretty cheap(10 pts), Str 4 2 shots and defensive weapon as it can be fired along with your main weapon. Sadly with only 2 dice at hand any damage this thing do is consider a bonus as u aint gonna kill MEQ or kill much hordes. However it good when laying on TOF( Torrent Of Fire). Torrent of fire mean is taking out enemies with quantity of weak str shots as oppose of few of high str and low ap shots. E.g Predator Destructor as it autocannon and heavy bolter is already dishing out 8 shot per tank so adding a storm bolter can add top it to 10 shots if range permit. If your army is generally aint into mid field combat then stormbolter may not be effective even as point sinks.
Hunter Killer Missile
For 10 pts you can have a single Str 8 Ap3 krak missile for your vehicles. The good thing about this is it grant almost any vehicle even a mere Rhino to knock out a tank or damage it. Another good thing it has infinite range so you can take out target that out of that 48". However the problem is that is a one shot weapon so any misses is consider a waste of 10 pts.
So the hunter killer issue is more on the do you have enough anti tank capabilities. If yes then getting it may just be extra redudancy , if no then it better to actually change your army list than to start spaming hunter killer missile fulfill that anti vehicle needs.
Is like a snow plow that allow the vehicle to re-roll any dangerous vehicle terrain test. Its pretty cheap (5 pts) which helps on moving though terrain so improve your odds when taking those terrain test. So you don't have to hesitate on take that shortcut route or have to work the long way around just to avoid the test. The question of getting it is more simply as are you constantly on the move, If yes then take it , if no then then dont bother..
It turn vehicle stun result to shaken. Which mean that now the only thing that can stop the vehicle from moving is a immobilize result. The great thing as mention it help vehicle to keep moving especially transport since after all its priority is to send the passenger to their objective and not shooting so as long as it can keep moving it accomplish its goal. The problem is that extra armor is expensive (15 pts) which only cover a 1/6 possibility for any both penned and glance result. It can be debatable that it make a huge differences with melta weapons as a melta weapon at its worse outcome stun a vehicle so a extra armor can take advantage of the unlucky melta attack and escape. Hopefully getting out of melta range. On the contrary any contact with melta weapon is usually fatal and getting extra armor for such an occasion is just waste of pts. So extra armor is useful for Land Raiders that carrying assault terminator as the last thing you want is your deadly package is stuck in the middle of the battlefield. Also it can be useful for walkers like Dreadnought as extra armor means u can still continue to move and assault.
This article is more to cover on Vanila Space Marine codex as other space marine codex something that Vanila user take for granted have to be purchasable by other codex. As the measuring the pros and cons of the option before buying. So here a few example for non Vanila Space Marine codex.
Blood Angels searchlight: Are you going to attack something at midfield regardless of circumstances. If yes then just get a few to cheap unit so they can drive close and give searchlight for your important unit to see. So even if the enemy hit back they can only take out your cheap unit or risk rolling 2D6 x3 just to spot your important unit. If no then dont bother, use the night to cover your men at a distant instead of attempting to go close and risk getting spotted.
Black Templar smoke: Are you gonna move your vehicle out? If yes then take it , if No then use common terrain for cover safe.