In Jain dharma, pachkhan is very important.

• If you intentionally take pachkhan and then observe the fast, it becomes a proper religious vow with full spiritual discipline.
• If you do not take pachkhan, then simply not eating may still have some merit, but it is not considered a complete or proper upvas in the Jain sense.
• The main benefit of fasting comes from sankalp, restraint, and awareness. Pachkhan gives that spiritual firmness.

So, yes, some benefit may remain from restraint, but the full Jain benefit of a fast is प्राप्त when pachkhan is taken.

In simple words: Pachkhan makes fasting a vow; without it, it is usually only physical abstinence, not complete vrata.