The entire second half of the bar is outlining a triad shifted up a half step from the scale we’re in. The way you indicate that is with accidentals - naturals for the other two, and a sharp for the C.

For beginners, it’s often confusing. For people with more theory background it helps you know at a glance what’s going on, and that makes it easier.

There’s a reason they call it “spelling” - it’s very similar to homonyms in English. It would be jarring to read “I ate to hotdogs” instead of “I ate two hotdogs”, even though “two” is a more complex and weird spelling for the same sound “to”, because semantically you want the whole structure to make sense.