Some buffing spells are only used once, and need to be used before the spell ends. However, Bless stays active for the full 10 rounds, meaning that every single saving through and attack roll you make during those 10 rounds will have a 1d4 added to it. This is very powerful and one of the reasons that a blessing spell holds up during the mid and even late game. Especially when you add it to multiple creatures rather than just the standard three. You just need to make sure that your caster doesn't get hit or knocked out during the battle, as the bless spell will go away. Keep them protected and make sure they have their concentration up! Keeping the bless spell up can really mess with your tactics as well, as you might need to play defense for your caster.