Why does the English add an "S" to 3rd singular fo

As we know it, every time in 3rd singular cases, in the English language, an "s" is added to the verb.
For example: The moon turns round the earth.
An "s" is added to the verb turn. That "s" has no substantial meaning in the sentence.
Why should it be so?

There used to be a different verb form for every person (for both singular and plural forms), but they all merged over time and now the only distinct form left is the one for 3rd person singular. So it's due to historical reasons. There is no meaning in the "-s" but you have to attach it anyway, or else you won't be speaking correct English.

<<Why should it be so?>>

Because that's the way it is. ;-) All languages have peculiarities that can't be explained logically.

"Because that's the way it is. ;-) All languages have peculiarities that can't be explained logically."

Well that's true. But this example isn't really one of them. As "Guest" noted, the "-(e)s" is a vestige of what was once a more developed verb conjugation system.

It should be pointed out here that the use of "-(e)s" is not simply restricted to the "3rd singular," it's actually restricted to the "3rd person singular present simple (indicative)."

<<But this example isn't really one of them.>>

Let me rephrase; "peculiarities that aren't logically necessary".

I have no idea, but for a native English speaker, the way " the moon turn around the earth" just sounds weird!

>>It should be pointed out here that the use of "-(e)s" is not simply restricted to the "3rd singular," it's actually restricted to the "3rd person singular present simple (indicative)."<<

Important note, as the presence of "-(e)s" is used to distinguish the present indicative from the present subjunctive in the third person singular in dialects which have not lost the subjunctive.