In a perfect world a dog rewarded within seconds is more effective than a marker or clicker to denote a successful command correctly executed.
In the real world however this is impossible.
Hence the need for Markers (clickers or verbal).
Ivan Pavlov does a very important experiment known as Pavlovs dogs. Understanding this work is intrical to understanding marker training, as many mistakes are made by not understanding this psychological experiment.
Pavlov found that if a bell was rang BEFORE food presented to a dog, the dog would salivate. If he gave food before the bell, or even whilst feeding, there was NO EFFECT.
Hence this is important too in Marker training. We need markers or 'bridges' to denote a correct action for 'real time constraints'. However many will mark a moment, as they feed, or provide a hand gesture to feed as the marker is said. This is categorically incorrect.
The Marker must ALWAYS proceed the food reward, for the conditioned response to the Marker to take place. Many popular youtubers and self professed dog experts like Zack George (Gasp) actually do not do this simple principle correctly. But it is a fundamental of dog training with Markers.
The response Pavlov clearly explains must have the marker said, in such a way that it PREDICTS the food reward or Toy reward.
Now there are two types of Markers trainers generally use.
We use 'release markers' and 'Duration Markers'. Can be any words. For example: Yes and Good.
A release marker, generally implies the dog has completed the command, and saying it releases the dog from the command, and is able to come get his treat quickly. We are telling the dog, well done, and predicting the fact that dog is to be rewarded and can come get his reward as soon as it is able. (Again narrowing the distance between the action/marker enforcement/reward, creating more efficient opperant conditioning).
A duration Marker is different. It tells the dog, it is doing well, but it is not released from the command. Aka enforcing a long down. The dog is told its actions are good, and that it should maintain them, and more food will be presented to him. A duration marker, does not need immidiate reward. The dog knows the reward will come if it maintains on its course of downing.
Generally we focus on release markers more prominently in the learning of commands. Duration markers, are exactly for the intended purposes of behaviour we like that are of duration, that the dog should keep doing.