For a few of examples: Two languages with some shared history will have a lot of words from similar roots. That makes the vocabulary of the new language easier

For a few of examples: Two languages with some shared history will have a lot of words from similar roots. That makes the vocabulary of the new language easier to learn. If the grammar is similar, it takes less practice to get used to the word order. If the grammar of the new language is more regular, it means fewer special cases to remember. And obviously, things will be easier if you don't have to learn a new writing system, or if the writing system is quick to learn.