here is how points work:
(letter tile value's added up) * word length.
so in tower mode, getting a blue tile on a 3 letter word isn't worth it because you're only getting *3 all those letter values that its removing. or it would be worth it if theres no other way to remove those letters since you're getting the lowest multiplier. whereas getting a 9 letter word with a blue tile would be really worth it, because you're getting *9 all those letters, and its unlikely you'd get that otherwise
also another tip. if you double tap tiles, you can mark them with a little yellow triangle. This is helpful if you're trying to set up a big word, you can mark all the letters you want to use in advance so you don't accidentally use them when you're manipulating the board