What is the purpose of eval? [duplicate]

Question

1. What is the purpose of running a command with eval?

2. What kinds of commands can eval run? I find it can run builtin command, external executable command, pipeline, script. Can it run all kinds of commands which can be run in a shell directly?

3. Compare running a command directly in a shell and indirectly via eval. Is the difference only in that with eval the command will be parsed twice instead of just once?

4. Compare running a command via eval and via command. Assume that the command can be run by both. What differences are between the two ways?

Answer 1

After the Command-line has been expanded:

eval sends back the arguments to the first step of "Command-Line Processing".

All steps of Command-Line Processing are presented in this page.