Almost everything is 8-bit
8-bit is the default for consumer capture and for nearly all web delivery. 256 levels per channel, about 16.7 million colours. It is what a phone shooting without HDR records, what a screen recorder produces, and what most stock footage and most finished SDR exports contain.
Check with the pixel format:
ffprobe -v error -select_streams v:0 \
-show_entries stream=pix_fmt -of default=nw=1 input.mp4
yuv420p is 8-bit. yuv420p10le is 10-bit.
What converting to 10-bit actually buys
Not more colours in any meaningful sense. The gain is specific: when a conversion stretches the highlight range to fit HDR's larger space, doing that at 8-bit would spread 256 levels across a wider range and create banding that was not in the original. At 10-bit there are 1,024 levels to spend, so the remapping is smooth.
In other words the extra bit depth is there to avoid a new problem, not to fix an old one. That is a real benefit and it is worth being precise about.
What stays broken
Quantization in the source is permanent. If an 8-bit sky already shows visible bands, those bands are the file: the values between them were never recorded, and nothing downstream can interpolate them back into existence with any honesty. Converting will not remove them, and stretching the range they sit in can make them slightly easier to see.
The same applies to clipped highlights. A blown-out window in an 8-bit SDR file is a flat area of maximum values. HDR gives that area room to be brighter; it does not give it detail.
The realistic expectation
An 8-bit SDR clip with clean gradients and highlights that are bright but not clipped converts well: the highlights separate from diffuse white and the picture gains depth. A clip that was already banded or already clipped converts to a 10-bit file with the same banding and the same clipping. Related: what conversion can and cannot do.
Questions people ask
Is a 10-bit output worth it from an 8-bit source?
Yes, for a specific reason: the conversion stretches the highlight range, and doing that at 8-bit would spread 256 levels across a wider span and create banding that was not in the original. The extra levels prevent a new problem rather than fixing an old one.
Will the file get much bigger?
Usually 20 to 30 percent, varying with content. That growth is expected and is a rough sanity check: a converted file noticeably smaller than its source has probably lost the highlight detail the conversion added rather than gained anything.