I’ve been told to upscale my photo before getting it printed
It is a recommendation. You do not have to if you do not want to. The point, in reality, is if you actually know how a 100PPI image looks like.
100PPI means that you have a little square of 1/100 of an inch. You might see them or not, depending on the viewing distance and the print quality. But also on the detail of the image. You normally see these pixels on a diagonal sharp line. They will look like a saw.
Upscaling it to the exact double, will give you 200PPI, which is normally enough even for close distance viewing. It will diffuse the pixels, which normally is less distracting than seeing the saw line.
Here is a specific example.
The dot on the top is the original size.
And on the second row, I used 3 different methods of resizing. The first one is just upscaling the original image, so you see the same pixels just bigger. That is what you would get, not only if you do not rescale the image at all, but if you could actually see the individual pixels, depending on the distance, but say around 100PPI.
The other two are using different resampling methods, In the second one, you can see the effort to blur the rough squares averaging the value of these new pixels. Probably the third method did a better job.
Some new AI methods try to guess what is the original directionality of a pattern and line. If the algorithm worked ideally, they would get something like the third row. Which in my case is only a fake example. I used a vector shape for the last row.
They are asking you to resample because they want you to make the decision on which look you want, in the case the pixels are distinguishable.