'They would be great FOR the concert' es como decir 'ellos serían buenos POR el concierto', en cambio 'they would be great TO the concert' es como decir 'ellos serían buenos AL concierto'. La palabra 'to' indica un movimiento y en ese caso es incorrecta.

"They would be great for the next concert." The two Spanish words "por" and "para" both translate into English as "for." That is, we use one word for both meanings. That's why I have trouble knowing whether to use "por" or "para" in Spanish--in English they are the same word.

for, not to