If you’re a black-and-white “This is what I want and everything else is WORTHLESS” type then I can see why tactical voting wouldn’t make sense to you. But I think everyone else understands it even if they don’t do it. If your candidate can’t win then you’re effectively voting for the bin, it makes more sense to vote for the candidate you most approve of, from the short list of potential winners. It’s the same concept that underpins STV and the like- you don’t love one candidate, and hate all the others equally.
I think last general election was the first time in my life I’ve ever had the “luxury” of having a choice of 2 parties I’d have liked to vote for, who had any chance. That’s fptp for you. I hadn’t even really given it much thought, and you know what? It’s shit. Really really really shit. The illusion of democratic choice.
But tactical does mess things up too, because if you have enough people going “my candidate can’t win, I’ll vote for someone else”, you can end up with perception basically sidelining a candidate who without tactical voting could have been in contention. This happens a lot in Scotland, historically- until recently the anti-tory vote often went to Labour and there were a lot of seats where the SNP could have been in contention, had so many potential voters not gone “don’t risk it”. My seat included! So as soon as winning a seat is perceived by the public as possible, you gain votes. That was one of the contributing factors to the landslide in the last general election- it wasn’t just voters switching allegience, it was also voters ditching their tactical vote. (more or less what’s happening to Bernie Sanders in the states; many people who’d support him have been convinced he can’t win the election, despite evidence to the contrary)
So the obvious problem is; if people become convinced a candidate can’t win, that can be the factor that stops them winning. That’s FPTP for you too.