I am certain of this ...
If a school you're applying to does not require the subject tests for admission, then you don't have to have them. It won't hurt you to NOT have them. In that regard, it might be considered a waste of time and money to take them. Most schools do NOT require subject tests.
But, there are schools that require them. They might require you to take "any two" or "a math and a science," etc. And then, of course, you must have the tests under your belt in order to apply at those schools. Typically, it seems the schools that require the subject tests are amongst the most selective schools in the country. The best engineering schools, ivy leagues, etc. If you're a junior now and think you may be applying to those types of schools, then you might want to get a few subject tests under your belt this year. Otherwise, I think there are only one or two test dates left in your senior year that will get your scores in on time.
My kids have come across schools that allow you to take the ACT with Writing OR two subject tests. And lots of schools require the ACT and/or the SAT.
You may be surprised how many schools do require the SAT (versus the subject tests). I think you'd be very wise to take that test between now and the June test date if you’re a junior and you haven't definitely narrowed down your college search. It would be a shame to discover a school you want to apply to that requires the SAT (so many do), only to realize you can't take it and get your scores in on time!
I'm sure you know ... you can find which schools require what by researching each college's admissions website page.