I just had one very small doubt...who said that magnetic field lines are obtained by joining the lines in which the compass needle points??? I mean why did we come up with this to describe the magnetic field ?

Well, to describe the shape or pattern of magnetic field, its better to use a device or an instrument that alligns according to the magnetic field and a compass needle is a best choice which alligns along the driection or tangent of the field. The same principle works behind when you sprinkle some iron filings around a magnet, where every single particle acts a tiny magnet.

 

Hope I cleared your doubt.