Because when you handwriting, the stroke order is very important.

Very important due to, tradition?

As a beginner Mandarin learner, my understanding is that historically, people wrote using the traditional stroke order, this informed what people think of as the aesthetically pleasing or "correct" way that the characters look. Now, if you want to write the characters in a legible and aesthetically pleasing way, the easiest method is to write them in the traditional stroke order. I think it's analogous to the way cursive writing in the west was taught, which informed the way it was written and what people thought of as the "correct" way to write cursive. If you wanted to learn to write in cursive, you could just look at existing cursive writing and try to copy it, but if for example you guessed that you should write it from right to left then you'd probably find it harder because cursive evolved to be written from left to right.

You can normally tell when someone uses the incorrect stroke order because things will be the wrong size. For example, when writing 因 you're supposed to write the outer ㄇ first, then the inner 大 and then the bottom horizontal stroke of the 口. If you start with the 大 then it's harder to write the outer 口 the right size.

Again, this is all from a beginner, so take it with a good amount of salt.

Writing by hand leads to optimizations, such as not lifting the pen a lot. This means that the movement between strokes also gets drawn. A 10 stroke character might end up being one single continuous path (effectively 10 strokes + 9 connections). Depending on stroke order, the results can be wildly different. So consistency makes sense for the result to be intelligible.

It's partly just that it's tradition, but there are also practical reasons:

1) Chinese characters are traditionally/historically written with a brush, not a modern pen or pencil. Because brushes don't create uniform lines, there is a connection between the specific series of movements and the final appearance of the character (sort of like calligraphy pens). Basically, inconsistent movements tend to produce inconsistent-looking characters, so an agreed-upon standard aids in legibility.

2) Various components (most notably the "radicals") reoccur across many characters. Having a (mostly) consistent set of rules for how each component is written and in which order aids memorization because you're not learning every new character "from scratch".

3) Stroke order affects how mechanically efficient it is to write the character, which can be a pretty big deal when some of the more complex characters are upwards of a dozen strokes.

One thing is balance. If you draw characters out of the typical stroke order, things will often look lopsided or have weird proportions. When written with the proper order, proportions look nicer.

For example, think about writing a capital A. It’ll look different if you draw the middle bar first or if you draw the outer two lines first. F will also look different depending on whether you draw the vertical line or one of the horizontal lines first. Try a Q with the little bottom dash before drawing the circle. It’s not only weird but more difficult.

The difference in these characters is subtle, but you can notice it with your own writing. Now instead of 3 strokes to write a character, imagine those with 15 or even 28 strokes. The odd balance and proportions have cascading effects.

One thing not mentioned in other comments is that stroke order also helps with software that wants to do character recognition.

If, for example, you are taking handwritten notes on an ipad, and want software to convert the notes into text... well, knowing the order of strokes and having an agreed upon order helps considerably over just trying to match shapes.

Digital dictionaries also usually have a "handwritten input" mode to look up a character, and that mode will also recognize characters much more accurately when input with correct stroke order.