There's two general reasons to save Rare Candies for later.

1. It takes more experience to gain the higher levels. As such, if you use Rare Candies on the early levels, you're pretty much wasting them as the experience they'd be worth on a higher level Pokémon is far greater than on a lower level one. Especially as you don't generally encounter Pokémon higher than level 70, which means your experience income doesn't quite reach all that high.

2. Your Pokémon gain bonuses to their stats based on fighting other Pokémon. This is what makes the whole "Trained pokémon are stronger than wild pokémon of the same level" point, is from this stat bonus. Generations 1-4, however, this stat bonus can only be realized when levelling up during actual battle. The system was changed in Generation 3 but this "must level during battle" factor is present in Blue.

   The amount of bonus stats you can gain does have a cap, though. If you've been thoroughly training a Pokémon since the lower levels (or if you just fed them a bunch of stat-raising vitamins, which correspond to the bonus stats), you'll probably have hit that cap by the time you should start using Rare Candies.

In general, the above basically means to train your Pokémon very well, and generally only use Rare Candies when you've gotten to such a high level that reaching the last levels is a taxing process. By that time, you should have maxed your bonus stats (unless you caught them at a particularly high level).

There's two general reasons to save Rare Candies for later.

1. It takes more experience to gain the higher levels. As such, if you use Rare Candies on the early levels, you're pretty much wasting them as the experience they'd be worth on a higher level Pokémon is far greater than on a lower level one. Especially as you don't generally encounter Pokémon higher than level 70, which means your experience income doesn't quite reach all that high.

2. Your Pokémon gain bonuses to their stats based on fighting other Pokémon. This is what makes the whole "Trained pokémon are stronger than wild pokémon of the same level" point, is from this stat bonus. Generations 1-4, however, this stat bonus can only be realized when leveling up during actual battle. The system was changed in Generation 5 but this "must level during battle" factor is present in Blue.

   The amount of bonus stats you can gain does have a cap, though. If you've been thoroughly training a Pokémon since the lower levels (or if you just fed them a bunch of stat-raising vitamins, which correspond to the bonus stats), you'll probably have hit that cap by the time you should start using Rare Candies.

In general, the above basically means to train your Pokémon very well, and generally only use Rare Candies when you've gotten to such a high level that reaching the last levels is a taxing process. By that time, you should have maxed your bonus stats (unless you caught them at a particularly high level).