That has to do with the "c" changing its sound in front of some vowels.
'c' is pronounced as the 'c' in church when it is followed by 'i' or 'e'. Ex cena (dinner) and cibo (food).
When it is followed by 'a', 'o' or 'u', then it's pronounced like the 'c' in cool (BTW, the same rules apply in English).
If you want to have the 'c' sound of cool but in front of 'e' and 'e', then you need to add 'h'. Ex. chi (who), che (that, which).
To obtain the 'c' of church in front of 'a', 'o' or 'u', then you need to use an 'i'. Ex. cioccolata (chocolate).
So, to answer you question, if yo just replace the 'a' of the singular with 'e' for the plurale, mucca (the 'c' is the one of cool here) become mucce (with the 'c' of church) which would be incorrect. Therefore you need to add an 'h'" to preserve the correct 'c' sound -> mucche.
The use of the determinative article defines something, to distinguish it from a non specific action or thing.
Le mucche bevono acqua is a general statement meaning that the cows drink water to survive, for exemple.
Le mucche bevono l'acqua means that the cows are drinking this very water and not the Chateau Margaux 1976 that they have graciously been provided :-)
Italian has two genders: masculine and feminine. Each has a singular and a plural form.
In the singular form:
- Masculine nouns usually end in -o (ex. il bambino, l'uomo, lo scolaro) and sometimes in -e (ex. il cane, il fiume, il televisore).
- Feminine nouns end in -a (ex. la casa, la scuola) and sometimes in -e (ex. la voce, l'elezione)
In the plural form:
- masculine nouns ending in -o in the singular: the -o becomes -i (ex i bambini, gli uomini, gli scolari)
- feminine nouns ending in -a in the singular: the -a becomes -e (ex le case, le scuole)
For both masculine and feminine nouns ending in -e in the singular form, the -e becomes -i (ex. i cani, i fiumi, i televisori, le voci, le elezioni).