Recently Jakarta got a flood disaster. In my mind, because I cannot directly help, I just chant and
meditate [on the] metta sutta for them.
1. Is it useful? How is the proof for that because in my opinion what I do is abstract.
2. How does loving kindness meditation function here?
Ananda, every action begins with a thought, good or evil (Dh 1-2). You were pained because you are compassionate. I'm sure there are many others (like myself) who share similar thoughts for these flood victims. Very often it is the poor and unfortunate who are worse affected by such calamities than others. So even if we are far away from them, the thoughts still count.
This type of meditation, where you channel your wholesome thoughts towards a certain area works better if you focus on a person or group of people you know well who are affected by the calamity. This way, the metta will be stronger. Try to be as clear as you can of their sufferings (news photos, TV reports help here). Acknowledge and visualize their sufferings and misfortunes, and fervently say aspirations like “I will help you! I will provide you solace and relief! May you recover from your pains and losses immediately!” etc.
These may be thoughts, but this is not the only misfortune people suffer. Every moment, there are people and beings suffering in some way somewhere. It was this realization that prompted Siddhartha to renounce the world to seek spiritual liberation.
Each time we do this spiritual exercise (radiation of lovingkindness), we are clearing the darkness and shadows in our minds and karmic past to energize ourselves with spiritual energy so that we are ready with wholesome responses when we are present in an actual bad situation or where others need help.
Most important of all, we are building up our spiritual perfections (parami). Gautama became the Buddha by at first merely thinking a good thought. Then he spoke the thought before a Buddha, and spent many lives perfecting various qualities (generosity, etc.). So all good actions begin with a single good thought. In modern terms: sow a thought, reap an action; sow an action, reap a destiny.
This is better that not caring, or worse, enjoying the misfortune of others.