GOTV's effectiveness is that you know you have specific voters who favor your candidate. With than info, and with real time updates as to which of them have not voted yet, a campaign can specifically contact those voters. The worst that happens is that they just don't vote, but some of the people you contact will turn out.
Sometimes they don't know where their polling station is; mine changed since the primary. Sometimes they think they need to bring the mailer from the county Registrar of Voters (they don't need to here in CA). Sometimes they don't know they can cast a provisional ballot.
GOTV helps to win elections, and is very cost effective in that it's largely volunteer driven.
Okay, I can see how personal phone calls from live humans can grab a few votes, as long as the callers are well-trained on how to approach people in a measured and friendly way. But the number of people who are retaining their home landlines is decreasing exponentially and therefore phone bank efforts will become increasingly less important in campaign strategery. And robo calls? Wow... They were an interesting curiosity back in the '80s, but now? Ughh.. A real irritation and, I maintain, actually counterproductive.
I also maintain that there must be better ways to utilize the energy of campaign volunteers than bothering people at home.