The sideways video problem
A phone camera does not physically rotate its sensor when you turn the handset. It records the frame the way the sensor sees it and writes a rotation flag into the file saying which way up it should be played. Well-behaved players read the flag and turn the picture; plenty of software does not, which is how a video that looked fine on your phone ends up on its side in an editor or on a website.
Rotating here does two things: it turns the actual pixels, and it clears the flag. Without the second step a player that does honour the flag would rotate the already-corrected picture again and put it back where it started.
Why cropping cannot be fast
The trimmer on this site is nearly instantaneous because it never looks at the picture — it copies compressed packets from one container into another. Cropping and rotating cannot work that way. Every frame has to be decoded, transformed and encoded again, and that is genuinely a lot of arithmetic. A minute of high-definition video is well over a thousand full frames.
Even numbers, and why the crop snaps
H.264 stores colour at half the resolution of brightness, which means frame dimensions have to be even numbers in both directions. An odd crop width fails the encode outright. The crop is rounded down to the nearest even number of pixels before encoding, so the result may be a single pixel narrower than the slider suggests — invisible in practice, and the alternative is an error message.
Choosing an aspect ratio
Cropping is the honest way to change a video's shape. Stretching 16:9 footage into a 9:16 frame distorts every face in it; cropping throws away the sides instead, which is what any competent editor would do. If the subject is not centred, move the crop box rather than accepting the default.