Just telling a story here....
I once used a host that offered "unlimited file transfer" and lots of other unlimited stuff. Not sayin' any names here. Really, no host can offer you "unlimited" amounts of certain resources for $4/month or $10/month or $40/month or more and make a profit.
So, when my traffic got up to about 1500 visits per day they were throttling my usage. They were not capping the bandwidth, instead they were capping database connections, or some other metric to keep me from running costs that were higher than I was paying them. Yep, I had unlimited bandwidth but I didn't have unlimited amounts of other resources needed to run a successful site.
At that host my sites were down a lot, timed out a lot. They didn't notify me that I was being throttled. They just did it. Instead my visitors noticed and I noticed and Googlebot probably noticed when he came to crawl my pages.
When I switched hosts, wow the traffic went up.
So, now I use hosts that charge me for every bit of resource that I use. They want me to be extremely successful and fuel my hosting with all of the resources needed, because the more successful they help me to be the more money they will make. And, I pay them a lot of money.
Most people on these "unlimited" plans never have a problem because they don't get much traffic... dozens of visitors per day or hundreds. But, if you have a site that is starting to become quite successful you might run into a situation where they need throttle your usage or change the agreement with you.
If you read BlueHost's "usage policies" you will see that they will throttle your service to protect their interests.
we do occasionally have to constrain certain accounts that use resources beyond what would be expected in the normal operation of a personal or small business website.
Read the full policy. I can't link to it, it opens in a window.
I really like my host because they look forward to me being successful because they want to charge me for all of the resources that I consume at published rates.