Translation:We can see the scientist because she emits photons.
If you put a comma in, it becomes 'otlhmey tlhuDmo' tej, ghaH wIleghlaH "because the scientist emits photons, we can see her".
The pronoun ghaH "her" is optional; you can add it or omit it as you like.
So the reason why it's there is because the speaker chose not to omit it.
One could also say wIleghlaH maH or even ghaH wIleghlaH maH for the second sentence.
Normally, we would try not to place two nouns next to each other like that - it makes you look for a conjunction or expect a genitive (or possessive) meaning. I would normally put a comma there to separate the two clauses and make it more obvious. In actuality, it is a great skill to develop to also begin to consider if the two nouns might actually be from two different phrases or clauses. This Skill is past the half way point, it might be a good time to make you practice that.
With ghaH omitted, the sentence could be interpreted as saying "Because it emits photons, we can see the scientist," or as "Because the scientist omits photons we can see it." The pronoun resolves that ambiguity, such that each clause has an explicit subject and object.
I see now that people are reading it as "Because it omits photons she is a scientist we can see it." Maybe I'll re-write it.