Apps and Android OS both continue to get more bloated as time passes, so the better the performance, the longer the device will remain relevant and useful. It's a measure of lifetime. A weak phone might not be able to run the apps that will be released two years from now. A strong phone might still be able to run apps four or more years in the future.
If you upgrade your phone every two years like clockwork, then it doesn't matter to you. Those who prefer to delay upgrading until its necessary will appreciate a phone that's powerful enough to age well.