it will pull the latest version of any base image(s) instead of reusing whatever you already have tagged locally
take for instance an image based on a moving tag (such as ubuntu:bionic). upstream makes changes and rebuilds this periodically but you might have a months old image locally. docker will happily build against the old base. --pull will pull as a side effect so you build against the latest base image
it’s ~usually a best practice to use it to get upstream security fixes as soon as possible (instead of using stale, potentially vulnerable images). though you have to trade off breaking changes (and if you use immutable tags then it doesn’t make a difference)