A video with lots of views and almost no likes reads as "people clicked and left". Likes fix that ratio and act as a ranking signal of their own.
A natural ratio on YouTube is roughly 2–5 likes per 100 views. So if you order 10,000 views, 200–500 likes is about right. Going wildly above that isn't harmful, but it looks less real.
Choose a refill service if you're boosting a video you'll keep promoting for months.