system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40K] Why do Space Marines serve in the Devastator and Assault squads -before- being made standard Tactical Marines? Shouldn't those be elite positions that they have to prove they are capable of and earn by first serving as a Tactical Marine?

Devastator Squads: Extremely heavy firepower that should only be placed in experienced hands due to the company's reliance upon it in crucial moments to turn the tide of battle and the danger of friendly fire it poses if wielded improperly. 

Assault Squads: Weaker ranged arms (pistols) that require greater finesse to use effectively, subjection to melee combat (and weaponry) which necessitates extreme martial skill and tenacity to survive, and the additional task of operating a Jump Pack all make this position considerably more complex than the Tactical Marine position that follows it. 

It seems to me that the roles of both Devastator and Assault Squads would not only require more skill (and perhaps bravery), but would also be of crucial tactical significance to the company/chapter on the field of battle and would thus logically be reserved for Tactical Marines who have proven they have the ability to handle such an assignment. Additionally, it is counter-intuitive to equip a marine with a Lascannon or Jump Pack and then take either away as a means of "promoting" him. 

So, why is it the other way around?

Edit: Thanks for the replies, guys! It all makes sense now.
gpt
Tac marines ARE the elite. As elite as you get before joining the first company. They can fill any role as required on the battlefield without hyperspecialised equipment. Either of those two roles has one real niche. Tactical marines? You need to know every possible role upside down and back to front first so when Brother Sergeant Chucklefuck and his merry men charge the Broodlord eating your Captain's face, you're not sitting there looking for the on button on your chainsword - or trying to get the hang of that whole predictive dodging thing that you were always told you'd need to remain alive versus something with claws that slice the universe itself and moves faster than an Eldar. Lives depend on you doing it and doing it well. Most marines start off as devastators to begin 'learning' the Battlefield - less need for conditonal awareness. And raptors don't need to be aware of anything beyond enemy positioning. A tac marine needs all of this and more.

Same deal with devastators. When the call comes through for covering fire, you don't want to be trying to puzzle out the best way to fire your bolter at something beyond nominal range. In both cases you have a few decades of experience in a specialised role to draw on in addition to your basic training.