1. It's the time. 20 weeks instead of 1 week is a big difference. Of course, if you're allowed to start the game w/ the trick and say you inefficiently trained the animal pre-game, less of an issue. Ideally you would get close enough where with an NPC hireling aiding you for +2, you can just take 10 and auto=pass the check in 1 week.
2. If the trick is trained to it already, the task you are attempting is "Handle an Animal", which is DC 10.
Handle an Animal: This task involves commanding an animal to perform a task or trick that it knows. If the animal is wounded or has taken any nonlethal damage or ability score damage, the DC increases by 2. If your check succeeds, the animal performs the task or trick on its next action.
3. It's a way to make money during downtime and how NPCs make money week to week. Presumably since the rules are written with adventuring PCs in mind, if you set up an actual shop/cart/whatever and reliably worked in one area for a long period of time, you could earn more than an adventurer that rolls in, puts in a week of work waiting for the next bounty to be put out, then leaves town to go monster hunting. Note that with Craft, actually making things to sell can yield more money than just half the check result per week. Still wouldn't be much more, though. Making money is much faster by looting monsters and their lairs, but also infinitely more dangerous.
4. Your Cleric knows Common, three additional languages from Int +3, and four more languages from Linguistics ranks. Common + 7 others total.