Most folks are used to positional numbers meaning they have a position to indicate the number of ones, the number of tens, the number of hundreds, and so on. The common Hindu-Arabic numerals are positional. "102" is one hundred plus zero tens plus two ones.
You can see why a zero is necessary. Otherwise "12" could be "one hundred and two" or "ten and two (twelve)" or "one thousand and two tens".
Many prior number systems don't work that way. For example, Roman numerals do the opposite. They use a different glyph for each magnitude, and repetition to indicate how many. "CII" is one hundred plus two ones.
There's no need for the numeral "zero". There's no ambiguity.
Similarly, Greek numbers have specific numerals for 1 to 10, but also 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, and so on. 102 in ancient Greek 102 is ΡΒ or rho beta. 100 + 2.
Still others, such as Chinese counting rods, were positional but had clear positions laid out on a grid or mat. 102 would be written as
| || with the space indicating an empty place. (You might be surprised to know that spaces to divide words are also a fairly recent invention. Reading was hard back then.) Later various glyphs were used such as the Zetian 〇,
| 〇 ||. These weren't "zero" but rather to clarify a vacant position.
Tracing the parallel evolution of the numeral zero is pretty well known and was a pragmatic thing.
The concept of "zero" as its own stand alone number has puzzled mathematicians and philosophers for a long time. To many, numbers were for counting and adding real things. Five sheep. Ten gold pieces. Two hundred acres. Numbers represented real things. These are the natural numbers. 1, 2, 3, and so on.
Saying "zero sheep" seemed weird. If you have no sheep just don't write down sheep on your list. How can nothing be something? How can you "have" no sheep? How can there even be nothing? In the classical view there was always something. There was always a medium to exist in, be it air, water, earth, or aether.
This is similar to a modern person wondering about infinity. Is infinity a real thing? Or just something mathematicians faff about with? But unlike infinity which is not a number it's a concept, zero has come around to being accepted as a number. Infinity is not a number. You can't add, subtract, multiply, or divide infinities (not without defining whole new number systems with new axioms).
While weird things happen around zero, like dividing by zero (which is not infinity), you can add, divide, multiply, and subtract zero. Including zero in your mathematical system makes many things easier.
The acceptance of zero as its own number is a bit more muddled. If you want to get into that, you might be interested in Zero: The Biography of a Dangerous Idea and Finding Zero: A Mathematician's Odyssey to Uncover the Origins of Numbers.