First is Hands On experience. Think of the R-Pi computer the same way an Auto Mechanic might have gotten started learning about cars, before cars became so complex. Or a Cook, or a Handyman, or a Carpenter; they all learned by doing. Starting small, reading directions, making things. They wanted to understand a part of the device or thing. So, "take it apart" and examine it, understand how it works, tinker with a part of it, put it back in place and see how it works better or not.
Another factor is Cost. You may have hear the term Open Source, that means the instructions a human can understand, that creates the code used by the computer, must be made available to the public to examine, tinker / modify and try on their our computer. For little or no cost. This means others will and can make changes to that code. If the code change they made is worth while, that improvement is made available to others to work with, examine and test. The worthy code survives, the poor code over time gets ignored, forgotten and destroyed.
Say you spend $75 USD on hardware (R-Pi, Keyboard, SD card, power supply, cables, USB Flash Drives, ...), without adding in the TV expense, that is dirt cheap for a tool that will expand his understanding of a concept built into every day life. New software and programming tools are available to download for the cost of an internet connection. The cost of unique books, on specialized areas will cost as much or more than the $75 of hardware.
Ownership & Community. Make it HIS machine. Not the family's or Mom's or Dad's, His machine. He can mess it up by accident and no one is hurt but him. He'll learn how to fix it, reload it, share with his friends. Except for the TV, it all can fit in a candy box or cookie tin very nicely. He can take it over to a friend house or to school in a backpack or day bag. He may play some games, he may make some games, he may someday discover that people want his help (for money) with other people's computers.
Think if his confidence & pride in understanding and creating software programs, or maybe building a special piece of hardware. He will learn to work on projects; taking the required steps, in the proper sequence, planning out the features and researching the answers to problems for his idea. These will flow over into other areas of his life, or not.
I hoped this answered some of your questions on why the R-Pi will be useful to a child.