Why do I need to cancel my Non B visa if it expires soon?

The visa must be canceled as soon as the task ends, this is the procedure

If the company says to come with them, then do so. In my case, it turned out a bit different, as I was working with an annually renewable non-B visa and then after some years stated my intention to leave one month before my anticipated departure from Thailand. At first the administrative staff said I would need to come with them to the immigration office to cut my extension short (about 1 months before annual renewal), and I filled out a form stating my intention to leave before the end of the contract and gave the staff my passport, but then they took care of it all at the immigration office without my needing to come with them or do anything else. I worried that maybe something had been missed since I didn't personally come to the immigration office, and so maybe they would fine me, but when I came through Thailand six months later as a visa-exempt tourist, they admitted me through immigration with a smile and no issues. But it seems like employees may normally need to come to the immigration office.

I doubt you're on a non-B visa. You are probably on a work extension, not any visa at all. If the stamp in your passport says "extension of stay" then you signed a document that said YOU would notify immigration if your circumstances change. If you stop working, that is a change of circumstances, and you are REQUIRED to notify immigration, regardless of when your extension ends. If you do not take your cancelled work permit and documentation about your work ending to immigration to cancel your extension, then you will be unable to get another long-term visa or extension in the future until you do cancel it, and you will be fined 500 baht per day, up to a maximum of 20,000 baht, for failing to notify immigration you stopped working.