Certain quests are only possible at specific times of RS gold day. For example, quests involving Vampires or night creatures could only be completed at night. Some creatures, like Vampires or undead, might not be accessible in the daytime. Morytania could be a deserted place in at day and populated with creatures in the night!
In the desert, night time can reduce the likelihood of being thirsty more often and walking around with one or none of the water skins will not be that much of a worry. Also, certain real world animals (like the crocodiles in the desert) could fall asleep at night and we would need to sneak past them or wake them up to fight them!
It is possible that combat can be affected by the time of night or day time in RuneScape. Certain Magic spells like the Lunar are more potent at night. Ranging can be challenging at night, as it's difficult to discern in the darkness.
As night falls, certain places like the wilderness and abandoned sections of the cities are prone to becoming extremely dark. This is when fire-making can be beneficial. In addition, there are many 'shady' people that are out in the streets and may attack us!
Ideas from Gonza Man! Day and night are completely optional. You will need to have a light source for those who travel that do not have a light source, e.g. lamposts. You'll need an illuminator, torch, or candle. In the evening, your energy will be exhausted rapidly (tiredness). There is a greater likelihood of being a victim of theft. Except for falcons, your chance of hunting will decline. Chickens, other NPCs and other animals should not have to be killed in the night. The wilderness should be free of revs during the daytime. Prayer is to buy RuneScape gold be continued until the night. Please let me know if have any other suggestions!