It is stated commonly that le is used in a series of bounded events. For examples,
1) 他吃了兩碗飯。 2) *他吃了飯。
tā chī-le liǎng-wǎn fàn tā chī-le fàn
he eat-LE two-bowl rice he eat-LE rice
In example (2), there is no numerical classifier before the noun phrases. The event is showed unbounded. Then the sentence is ungrammatical. However, it is not an issue about le, but it is about informativeness. Here is an acceptable discourse,
Q: 你昨天吃了什麼？ A: 我吃了飯。
nǐ zuò-le shíme? wo chī-le fàn
you do-le what? eat-le meal
‘What did you eat yesterday?’ ‘I had meals.’
