When we say "it's raining", what is the "it" that we're referring to? In fact, "it" doesn't exist - it's what's called a dummy or non-referential "it". Its function is grammatical rather than lexical.
"Grammatical" refers to meaning signalled by such things as word order and other devices like "s" to denote a plural. "Dog bites man" has the same words as "man bites dog", but the meaning of each sentence is different thanks to the different word order. That's grammar in action.
Every language has its own grammar. "Dog bites man" is a classic English subject-verb-object construction. Japanese, on the other hand, uses the subject-object-verb order - "man dog bites". In this respect, English is outvoted. In the 1980s, linguist Russell S. Tomlin reported that 45% of languages prefer the Japanese order, while "only" 42% prefer the English way. Hebrew, Irish and Zapotec opt for verb-subject-object - "bites man dog". Persian, Romanian and Finnish don't have a strict word order except in specific circumstances.
You might notice that every sentence above, regardless of word order, is in agreement about what each individual word means. "Man" means "man", no matter where you put it. This is what is meant by "lexical meaning", also called denotation, also called the dictionary meaning of words. "Bites man dog" is full of lexical meaning, even though the sentence itself is jibberish to an English speaker.
Which brings us back to the dummy or non-referential "it". It lacks lexical meaning - that is, it doesn't refer to anything "out there" in the world. But that doesn't make it empty. "It's raining" tells you something.
The dummy "it" behaves differently in different variations of English. In some creole languages such as Gullah, rather than say "there's a spider in the shower", a speaker would say "it's a spider in the shower", replacing the dummy "there" with a dummy "it".
Having defended the dummy words, now let me get out my shotgun for a moment. Because it requires so little thought, the dummy "there" is a common feature of weak writing. If your writing is stuffed with sentences like “there are five roads leading into the city”, you're placing a lot of empty words in your prose. One way to become more concise and energetic at the same time is to revise them to read “five roads lead into the city”.