In a perfectly functioning system, they give obviously guilty offenders a more lenient sentence in exchange for admitting guilt.

laughs hysterically

In the real world, of course, they are used to throw the book at people with no ability to defend themselves.

In a perfectly functioning system there is no concept of an obviously guilty offender because an obviously guilty offender cannot exist, because 'obvious' does not exist.

The only circumstance that holds true is if you get a legit confession. But that confession still has to be fact-checked.

As for the rest, you cannot in any circumstance be obviously guilty unless you were an accomplice to that fact or you were captured after the fact. You would have to read minds to throw away the presumption of innocence, and even in those seemingly obvious circumstances there may be something else going on. So we have the trials.

Plea deals are exactly that - deals between you and the state to go to jail so you might not go to jail longer. They're not doing you a favour, because they have all the authority.