Imperial Mandate is intended to be a mage support item (champs like Brand, Swain sup, Lux, etc.). Furthermore, ADCs are in very rough spot now, making mage botlaners more meta. Imperial Mandate itself is geared towards the 2v2 (what with its double proc), meaning that any botlane Swain can utilize the item extremely well.

Swain APC especially benefits from Mandate for a few reasons:
[*] Grants a ton of mana sustain, allowing you to both Q-spam the wave and E/W spam enemies to stack souls quickly
[*] You have very easy Mandate triggers with Passive and W, especially with an engage support.
[*] Mandate deals an added 120-200 bonus damage to every enemy champion, EVERY 6 SECONDS. Compare this to Night Harvester (125-200 + 15% AP bonus damage EVERY 40 SECONDS), and you have comparable damage on a monolithically smaller cooldown.
[*] Mandate gives both you and an ally +20% movement speed for 2 seconds, which is AMAZING in the 2v2.
[*] Mandate is cheap - 500g cheaper than the typical mythics that Swain uses (Rift Maker, Night Harvester, etc.), meaning you can hit you 2 item spike much faster, and have a much stronger level 6/ level 11.
[*] Mandate removes your mid-late game reliance on Blue Buff. When I used Rift Maker, I was constantly running out of mana, and would have to recall after every single fight. Mandate + PoM means your mana woes are no more, meaning you can actually help take Baron after you 1v9.

Imperial Mandate is also a great item for Support Swain, but who plays support? Queue up for some Swain carry, it's infinitely better than every ADC in the game right now, especially after you and your support ban Jhin and Samira.

Some people think that damage sources are the most important part of the game, others say resistances are the most pivotal part of League.

They are all wrong. The most important statistic in League of Legends is Movement Speed.

Ghost gives you the most important stat in the game. It scales into teamfighting incredibly well, it has a decently short cooldown, and it means nobody can run from you OR catch you. Play 10 games with ghost, and you will never want to play without it.

By not taking heal (the summ I usually take on ADCs), you are sacrificing 90-345 eHP for two champions every 3 minutes for a massive boost of movement speed that resets on takedowns. You massively improve your kiting ability, you give yourself more E angle options, you can keep your R applied on multiple targets for longer... The benefits of taking Ghost are endless, and Ghost + Phase Rush means that you ARE speed.