The short answer is that during the interwar era, tank design was focused on an anti-infantry role to a large extent. To this end, almost every tank developed during the interwar and early-war era attempted to provide 360-degree machine gun cover. As the war progresses, you see machine guns removed from tanks for ease of production and maintenance. If you're looking for some good examples of this trend, I want to point you to some visual displays being created on /r/tankporn by /u/TheHIV123, who has been doing some great explanatory work about how some of World War II's most famous tanks were designed. Check out these links:

• The Stuart light tank
• The M26 Pershing
• The M3 Lee/Grant
• The M6 Heavy tank

He's written plenty more, and I encourage you to check out his work for a good basic look if you don't have textual sources handy.