(I know I've already replied to you on another SD but leaving it here just in case someone else also has this question)
It's a different tense
Simple present: मैं टमाटर खाती हूँ/I eat tomatoes - Denotes a habitual or regular action
Present continuous: मैं टमाटर खा रही हूँ /'I am eating tomatoes' or 'I am eating a tomato' - Denotes an action happening currently
हूँ is the equivalent of 'am' but usage is different. In Hindi, it accompanies all present tense sentences, not just in the continuous aspect.
It's the same in Hindi.
मैं टमाटर खाती हूँ is the simple present tense sentence and it denotes a routine, regular or habitual task.
Eg: मैं रोज़ नाश्ते में टमाटर खाती हूँ - I eat tomatoes for breakfast every day,
मैं टमाटर खाती हूँ लेकिन मुझे वे अच्छे नहीं लगते - I eat tomatoes but I don't like them.
मैं टमाटर खा रही हूँ is the present continuous tense sentence (I am eating tomatoes) which is used when you are eating a tomato at the present moment.