because, first, if your handwriting is not very beautiful.
2. If you want to send a lot of letters. Don't be exhausted from writing.
3. The contents of the cover should be consistent with the letter, which looks more comfortable. (Either it's all handwritten or it's all printed)
Before, it was handwritten, because there was no machine. If you wrote all by hand, it would make the other person think you were too stingy.