The change in the words isn't the important part - tracing the changes through time is how we arrive at the original PIE word. The important part is that the words were conserved throughout the daughter languages which indicates that the original PIE speakers had words for them and the were used enough and important enough to be passed down from the generations. A lot of really common words in IE languages can be traced back all the way to the hypothesized mother tongue simply because they are commonly used words. The fact that there are tons of preserved words relating to horses, chariots, and wagons tell us that the original PIE speakers likely used them a lot.