It's painful to do alchemist and leatherworker because they require monster drops, while almost all other classes just require you to go out and grab shit with miner/botanist/fisher (and fisher only for like a couple of things in cooking). Retainers can make those specific classes dramatically easier.
Really, if you've not dropped money each xpac doing xpac things in gathering and are just leveling now, you should have tons of money. Just go buy whatever mats you need if you don't want to gather haha. If you've spent money on like... exarchic stuff (current endgame crafted) then you may be brokeish so yeah, gatherers ahoy. Very little should actually be a pain, minus getting like darksteel ingots/other "rare" gathered. And I would say to make your life easier, check out one of the rare spawn sites (
FFXIV Clock) has always been my goto, and just farm up the things you need if you don't want to spend money. And if you do that -and- have like miner/botanist (fisher is.. it's own thing) you can then send retainers out to gather said rare things.
edit - I will say, the nature of the game only really requiring the single character, it makes it easy to take the "lighter" initial cost account deal that only allows 1 per server (or datacenter, can't even remember) and grab a couple extra retainers and it ends up still being 15 a month. Make one a botanist, the other a miner, and then whatever is your highest combat class for the others (can mix it up but that just means more equipping etc since you tend to get extra for other stuff but only one real set for your main). Leveling can take a minute (you'll need to level those retainers over time, and the gatherer ones won't be able to go for shit you haven't gatherered) but they pay off massively in the end.