I believe it was in primary school I was taught that a story has a beginning, a middle, and an ending. This is sadly not always true these days. I've had the bad luck to run into many novels that cannot claim to fulfil the requirements of an ending. They left me stuck in a difficult situation right along with the protagonist, with no hope of getting out unless the author publishes another book someday - which I am however now unlikely to buy.
It seems to me that a lot of publishers are staking their bets on first-time authors who will need at least another book out to get their stories finished. But what happens if that author turns out to be a one-book wonder? What if the publisher goes under or cuts a genre in our world of uncertainties?
I do understand the writer's dilemma. It's happened to me too: plan a novel carefully, with its climax and resolution, then write and write and write until you realise you've written enough words but only reached a mid-point in your plot. When I realised this was the case, I went back and re-did the planning, so that a significant subplot could be introduced and woven in earlier on, finding its resolution at the end of Book One without leaving the reader hanging - even though the story goes on. It seems to me that many authors are not bothering with any attempt at a conclusion. The story just stops, suspended in mid-air, and I feel like throwing it across the room.
The end of a novel, in my humble opinion, is required to provide mainly one thing to its readers: Satisfaction. If it doesn't do that, what's the point?
That's why I always ask for opinions on reader satisfaction when I send out manuscripts for critique. After plot continuity and correct grammar, this is my most important criterion for technical quality as an aspect of planning. It has nothing to do with more inspirational aspects such as description, character, and emotional impact. Your writing style may be amazing, but if you don't resolve at least one of the book's major issues by the time it wraps up, you risk leaving your reader with a sensation of emptiness and unrest. Please, please, please don't do that.