Well explained by Great Guy in a very short space. Yes, you need to imagine yourself in the shoes of your reader and have to foresee what the visitor (we call them traffic here) can look up for in the search engines. Apart from the subjects mentioned above by Great Guy, you can also post on various "helpline articles" people often search for different areas. You can post about specialized doctors in a given area / city (like Cardiologists, Gynaecologists, Orthopaedics, Child Specialist and so on). Some other good topics are ambulance service providers, NGOs, coaching centres, competitive exams coaching centres and so on.
You can also post a how to reach Kolkata series (or some other place) from different parts of Bengal and outside. For example, some good topics are Mumbai-Kolkata flights, Delhi-Kolkata superfast trains, Kolkata-Siliguri express bus services and so on.
The bottom line is, as Great Guy said, imagine yourself as your reader!