Using articles (def or indef) before nouns helps to specify the gender and to indicate whether the noun is plural or singular - i.e if you were to say je mange fraises - it is not clear whether you are eating one or more strawberries (as the s is silent), but if you say je mange des fraises it is clear that it is several rather than une fraise .....

Je mange des fraises means I eat some strawberries, as well as I eat strawberries. It is better to place an article before a noun for style. Je mange fraises is incorrect.

des is there because you need an article before nouns in french. Je mange des fraises = I eat ( arbitrary) strawberries. An alternative, "je mange les fraises" = I eat the ( specific) strawberries.

"des" can be explained in two ways: 1. the plural form of the indefinite article "un/une". 2. the contraction form of "de les". In your question, we can say that it is the first case. So just remember that in French, you always have to put an article( un/une/des/le/la/les) before a noun. However, in some cases, we only have a "de" before a uncountable noun or a countable noun in plural form. For example: in the negative form: "Je bois du('du' = 'de le') thé, mais je ne bois pas de café." Some other examples are with the expressions ending with "de" like "beaucoup de", "trop de": "Il y a beaucoup de gens dans la classe." "J'ai mangé trop de viande."