One of the most important aspects of in-game strategy is flexibility.
Players who spend all of their resources run the risk of being inflexible when they need to confront situations on the battlefield. Oftentimes, successful players are able to find the right balance between holding resources and spending resources.
This balance will allow you to keep your military moving forward, collecting more resources and growing at a healthy rate. But more importantly it allows you to maintain sufficient resources to react if the need arises.
It cannot be stressed enough how important it is for your army to continue progressing and evolving.