German word order is not as strict as English word order. You may place subjects and objects as you like. The case markers tell the reader if a noun is a subject or an object, and which type of object.
German has no case markers on nouns apart from some exceptions (genitive singular and dative plural of some declination classes). Instead, it puts the case markers on the words describing a noun further. Adjectives, pronouns, number words. The indefinite article in German is identical with the number word »one«.
If there is no such describing word in front of a noun, you still need a case marker most times. That's when German puts the definite article in front of it.
Whereas, in English the definite article is only put in front of a noun as a pointing finger —a small demonstrative—. This use does exist in German, too, but it's not the only use of the definite article.