What RAM changes
RAM is working memory used by active applications and data. More RAM mainly helps when your workload is pushing beyond the memory you already have; it is not a universal speed upgrade.
When 8GB can still be enough
Light browsing, basic documents and simple everyday tasks can fit within 8GB, especially when you keep fewer heavy applications open at once.
Why 16GB is the safer general choice
For heavier multitasking, many browser tabs, programming tools, creative work or a laptop you plan to keep for several years, 16GB generally gives more headroom.
Before paying for more
Check whether the laptop's memory is upgradeable or soldered. Also consider the CPU, storage and GPU because slow performance is not always a RAM problem.