In the UK tax on interest earned on plain savings accounts isn't deducted at source - so if you have a rainy day pot chances are you're required to register for self assessment and pay tax on it (particularly now that interest rates are higher and it's relatively easy to go above the tax free threshold, which has been frozen for a long time).

If you have investments outside of an ISA (tax free investment wrapper) then same story - you need to report disposals and dividends for tax purposes.

That's before we get into side hustles/self employment and investment properties, etc.