Sometimes a vote for an MP is motivated mostly by the party that the candidate belongs to, sometimes it is mostly the individual, or a combination, but often a major motivation is ensuring that one particular candidate does not win. Each motivation for voting is legitimate, don’t let anyone tell you that you are voting incorrectly.
So how easy is it to get rid of your MP, or to ensure that a specific one doesn’t get elected? In the current electoral system you must do two things. First, convince at least half of the people in your constituency not to vote for that person. Secondly, if there are more than two candidates, you must convince most voters to vote for the same other candidate, that is to say convince many of them not to vote for the candidate that they like best, but vote for a different candidate for fear that the less liked candidate might win. That is difficult, because others who have the same motivations as you may be doing the same thing, but for a different candidate. It is entirely possible that, with 70% of voters agreeing that not electing one of the four candidates is their top priority, this candidate may still win with 30% of the vote.
The problem is even more severe when constituencies are large or when there are many representatives per constituency. For example, if the entire country is a single constituency, as happens in many countries, MPs can get elected with as little as 1% of the votes of their constituents. In those cases, even if you succeed in convincing 99% of the entire population of the country to vote for someone else, that is not enough and that person would still be your official representative.
Ranked ballots ensure that when a majority of constituents do not want a particular MP, they do not get that MP. There is no need for people to agree in advance which other candidate to vote for, and there is no risk that a widely disliked candidate will be elected, because the one you don’t like cannot win unless 50% of the constituents vote for him or her.