Here are some analogies I use for base building.
Its like building a foundation for a house. The stronger the foundation the stronger the house.
Its like putting money in a bank. The more you put in now the more you can withdraw when you need it later.
Also, some thoughts on base building. The better the base the stronger and more controlled you will be in the middle of the race.
For example, say an athlete has the potential to run a 4:40 mile race. He/she may go out in 70 seconds for the first quarter which is rate on pace. Now if your base is weak most likely the athlete will drop to 75 to 80 seconds for the second and third laps. Then even if they pull off a 70 second last lap they will still be far off that 4:40 time. However, if your base is strong you can go out in 70 and hold 70 throughout the entire race to run a 4:40. Its really as simple as that.