It's more... when you write code, you're telling a computer what you want it to do, and there's an interpreter who translates those instructions from code that a human can read and write, to machine code that the computer can understand.
That interpreter is whats looking at the dictionary; x86 has a huge dictionary that covers everything, ARM only has the most commonly used words. Because theres fewer words to look up, its a bit quicker to look up each 'translation'. But when thats being done millions of times a second, that 'bit quicker' adds up each time.
Its not a great metaphor... but if you think of the convenience of a pocket dictionary over a full dictionary, thats the sort of benefit involved; it can be smaller, it takes less 'power' to carry around, or to scan through, and its more efficient because it only has common words listed