What it is
ROA scales profit against the entire asset base — cash, inventory, factories, intellectual property, the lot. It's a measure of asset efficiency.
When it's useful
ROA is especially relevant for asset-heavy businesses: banks, airlines, manufacturers. They have a lot of capital deployed; the question is how productively.
Software companies, which have very few physical assets, can show enormous ROA numbers that look impressive but aren't very meaningful — the model is just light on assets.