In addition to what Miller said…
Merchants - the farther away you send them, the more profit they generate ( assuming a valuable resource ). So if you’re Milan, an excellent source of profits are the silks around Constantinople. But if you’re the Byzantine empire, those silks won’t generate much for you - but the cotton textiles around Milan will.
Generally textiles, wine, ivory, precious metals are all excellent sources of income if you trek far enough. You’ll notice that some “resource deposits” are much more valuable than other seemingly identical deposits. Thus Bordeaux wines are worth more than some other booze producing regions. A particularly rich bonanza of trading income, perfect if you play the Moors, are the trans-Saharan gold, ivory and slave resources scattered around Timbucktu and Arguin.
Merchant guilds and their improvements churn out progressively more experienced merchants. The longer a merchant sits on a single resource the higher his skill will rise. Elminating other merchants also nets you skill advances.
So playing as Byzantium ( recently ) I’d pump three star merchants out of Constantinople with its advanced merchant guild, sit them on a silk resource for twn turns or so to gain another favorable trait and pounce predatorily on any weaker foreign merchant that approaches to further up my skill. Then eventually I’d send them marching off to farm the gold mines in the balkans, the silver mines in Bohemia, the textile emporiums of northern Italy, etc.
I find merchants are more useable post-patch, but the key is aggression. If you see a significantly weaker compeitor ( i.e. better than 50% chance to out compete him ), wipe him out. The stronger your merchants are, the more income they generate ( can be a LOT - multiple hundreds per turn ) and the more resistant they are to foreign predation.
Priests - In addition to building better buildings, parking priests in areas where they will do a ton of converting will cause their skill levels to rise very rapidly. It’s one of the payoffs to crusading. Your priests in the Levant are going to gain piety much quicker than those guys in Germany and thus will start filling up the College of Cardinals. Cardinals cannot be lost to heresy and if you elect the Pope you automatically get perfect relations with the Papacy ( for a time ), which can be very useful to a Catholic faction. Poor relations and Catholic = increased inquisitor activity. Which is bad ( if not as bad as pre-patch ).