Because most people can't read English.
If the world's best encyclopaedia was only available in Swahili, would you still be asking "Why actually spend all this effort on localised articles?", assuming of course that you're not a Swahili speaker.
That's a good argument for some languages, but for Swedish in particular it's less strong, because the number of people who can read Swedish but not read English is fairly small, and rapidly declining. That doesn't mean there's no point to a Swedish Wikipedia, but whatever point there is can't really be because Swedes are unable to read English.
That's also true of Danish, which is one reason I believe the Danish Wikipedia has been languishing. There are many Danish people who are Wikipedians, but many of them do more English editing than Danish editing. The Swedish Wikipedia seems to be more active in comparison. It would be interesting to understand why; is it just the population difference (9.5m vs. 5.5m)?
As a swede I use both swedish and english Wikipedia. It depends on what I'm looking at. Swedish Wikipedia contains more in depth content on a lot of topics.
People shouldn't want to read articles like the one about cryptography (just picking something technical at random) in a non-English language beyond what the word means and a general overview. There are so many English resources and so many people able to read English if you'd contribute resources, learning it should be encouraged rather than translating everything just because we can.