The proliferation of 'दादी Smith' apples in Indian markets is a relatively new phenomenon. Unfortunately, much of the country is still under the tyrannical yoke of imported 'लाल delicious' apples (apart from the few local cultivars from the Northern states).
It is used to make generalizations or talk about universal truths.
The sentence सेब लाल हैं may be talking about some particular apples in front of you. But with होते, the sentence सेब लाल होते हैं is a general statement about all apples.