In addition to what's been said already a bit of background information:

You can modify the crontab-file directly, but that will not work be enough.
You have to change the timestamp of the directory in which the crontab-file resides, because that's what crond checks.
And that's what the crontab command does for you. It changes the crontab-file, either by replacing or by opening an editor for you, and it 'touches' the crontab-directory to inform crond to reread that file.