Using just "em" makes it indefinite, but when it's about "casa", people assume that "em casa" is talking about the house of the referred person. Then, if you say "ele entra em casa", it means: he enters his house.
Also, the word "minha" determines which house is it. Making it unnecessary to use the article.
But in other cases there would be differences:
- Ele entra na padaria = he goes in the bakery
- Ele entra em padaria = he enters bakeries. (indefinite)