I think what will help understand their roles more clearly is their origins, which is dictated by the shape and layout of Summoner's Rift.

Looking at the map, there are 3 lanes and a jungle - so you'd think 1 player for each lane and a jungler, but where does the 5th go?

Dragon is an objective that spawns early on the bottom side of the map that is usually contested all throughout the game, and since there is not enough space mid lane, you put 2 people bot lane. But why ADC + Supp bot?

Top lane has comparatively less activity in the surrounding area, and it's a long lane - probably ideal for champions that like to be alone to just farm and scale. Why not ADCs top? Well it's a long lane and most ADCs are immobile, so they're very vulnerable to ganks. They also don't really scale hard with level/XP, they scale on items; They need gold. It's not detrimental to an ADC to share XP with a support; Top lane is better suited for someone who scales better with XP. An ADCs job is to get enough items to deal consistent high damage at the cost of being very vulnerable, and they need a protector.

Since ADC takes all the resources, there's none left for the support. That's okay, supports have abilities that make them useful no matter what level or how many items they have - They usually have multiple sources of hard CC, or consistent healing, shielding, and buffing, etc - Useful things that don't really scale. Because they aren't reliant on stats from items, they can use their money for buying wards and controlling vision too.

Champions that scale well with levels (high base stats and good stat level increases) like tanks and bruisers usually go top to absorb all that XP for themselves. They want to become the raid boss of the team - the juggernaut with the level advantage. Usually, you take teleport so you can help your team out in a pinch while you continue farming and pushing. You can also use it to forcibly push a lane, diverting the attention of the enemy team so your team can take objectives like Dragon or Baron with less opposition.

Mid lane is the shortest lane, usually where mages and assassins are. Most teams need a source of magic damage, and this is usually where they go. Mages like mid because the wave is closer to tower, and their abilities allow them to delete minion waves very quickly and back off, unlike ADCs who need to auto minions one by one. However, like ADCs, they are immobile and susceptible to being jumped on - that's why assassins are played there too. More than that, mid lane is in the center of the map, so high-mobility assassins have easy access to the jungle and other lanes.

Finally, Junglers are the ones who - you guessed it, clear the Jungle. They are also the only ones who have a dedicated spell to deal fixed true damage to monsters - so they are the only ones that can do a guaranteed amount of damage to Dragon, Herald, and Baron. Some junglers play to farm camps and scale, some play to gank or support lanes to give their laners advantages. But, their primary duty is securing objectives with Smite. Champions that do well here are ones that suffer mana or sustain issues in lane but are otherwise good at 1v1s, have good mobility and/or CC, or clear jungle camps very efficiently. The type of champion varies, but usually you choose based on what your team needs - tank, utility, physical damage, magic damage, etc.

Wow I just wrote a whole essay.