When driving on the road during rain, it is always advisable to turn on your lights. Road grime, poor quality or worn wipers, or a poor quality windscreen, in addition to rain mist, and water spray thrown up by other cars will affect your vision.
You will find it difficult to see other cars on the road, and similarly, they will also find it difficult to see you. More so if you are driving a dark-coloured car, and even more so if it is near dusk or when the sky is dark due to heavy rain clouds.
Another thing to note is that many modern cars have instrument panels that are lighted up whether or not the headlights are on, and so we tend to forget that our lights are not on. Usually there is a bulb icon on the instrument panel to remind us if the lights are on. Whatever the case, make it a point to turn on your lights whenever it rains.
The lights are for others to see you and be aware of your presence. Drive safe!