Normally, nouns need a determiner such as an article.
Numbers, however, can act either as nouns or determiners.
We can say
Three is two more than one.
Three is a positive real number.
In those cases, "three" is being used as a noun.
We can also say
There were three thousand people in the room
In this case, we are using "three thousand" as a determiner to specify the number of people being discussed.
However, English does not normally have a way to modify determiners.
So if we have an estimated number, it seems unnatural to an English speaker to treat the number as a modified determiner. But it also seems unnatural to not have a determiner at all. And determiners precede adjectives: we do not say
ball the red.
So we essentially add an extra determiner so that the adjective comes where we expect it to, after a determiner.
Three thousand people
An estimated three thousand people.