In Farming Simulator 19, a good combine harvester is determined by two parameters - its power and capacity. The first stat determines how big attachments you can use and be fully functional; the second parameter determines how many times you will have to empty its container. You can do that in two ways - drive a tractor with a semi-trailer and place it right below the harvester's pipe to empty its container; you can also drive your harvester to the nearest semi-trailer or go straight to a silo.
Your combine harvester can also tow a semi-trailer. However, you can only use auger wagons. This solution has its pros - these wagons have the same pipe to unload yields that is used in a harvester. You won't be able to do that with regular semi-trailers. Each of the four auger wagons is doing its job - they differ in capacity and price (they also have different weight - don't buy the heaviest model for a tractor that has a weak engine!).
You need to detach a semi-trailer before you unload your yields - however, you only need to do that for a brief moment. You can't rotate a semi-trailer more than 90 degrees in relation to the combine harvester without detaching a semi-trailer. After a bit of practicing, you will do that in two moves.
Tip: Bizon Super Z056, the cheapest combine harvester, has the option to attach every light and cheaper semi-trailer.