What uses download?
Streaming video, browsing, loading apps, downloading games and files. This is most of what people do online, which is why plans emphasise it.
What uses upload?
Video calls (you are sending video), live streaming, cloud backups, posting large photos and videos, sending big email attachments and online gaming uploads. Upload matters more than ever for remote work.
Why are they different?
Cable, DSL, mobile and satellite connections are asymmetric by design because traditional usage was download-heavy. Fibre plans are often symmetric, giving equal upload and download.
How much upload do you need?
Roughly 3 to 5 Mbps covers an HD video call, 10 Mbps or more is comfortable for a household with several calls, and 15 Mbps or more suits live streaming in 1080p or heavy cloud backup. As a rule, if upload is more than about ten times lower than download and you upload a lot, a symmetric plan may be worth asking about.