Is the only way to earn points tiles and donation tiles, through coworkers in the office and the sponsors enclosure?
Are points tiles only used for victory points at the end of the game? if not what are they used for?
1. Yes. Point tiles are used for those circumstances in which something can earn you points repeatedly. In this case is exclusively due to the sponsors enclosure. The coworkers office that has the point-tiles image just enhance this enclosure. The sponsor enclosure gives you 1 donation tile when completed, and there is a coworker office that gives you 1 donation tile every time you fulfill an enclosure that gives you money. (Only 1 tile regardless of the number of coins the enclosure grants).
2. Point tiles are counted at the end of the game, and are added to the total of points you earn for other things. If you notice, previously (to this expansion) everything gives point just once, at the end, if the given requirements are met. But now, the sponsors enclosure lets you earn points repeatedly, hence the need of point markers (tiles in this case)
Also is worth noting that this expansion is really designed to go with the previous two. If you play without XXL, it is really hard to make use of the sponsors enclosure. If you play without exotic, well you have a very weird looking puzzle piece on your board.
That said, this expansion does not include a market tile for the fifth stand added in XXL, which I consider is really a bad design decision. Specially when you consider the huge amount of blanks (green "spare" tiles) that this game comes with.