Eagle has different shape for pads for through hole components why?
As well as being useful for marking particular pins, a square, octagonal or oblong pad has more copper area than a round hole of the same width. Through hole components such as connectors and large capacitors often have significant mechanical stress placed on them while the device is being operated or repaired. A larger copper area is better able to resist the track from being torn off the board, and the increased amount of solder on the larger pad also makes the joint stronger.  

The pads on DIP ICs are often elongated to get maximum surface area while still allowing tracks to go between them. A multi-row pin header may use hexagonal or square pads instead, because there isn't enough room between the rows for elongated pads. Hexagonal pads provide a bit more room for routing tracks at 45&deg; than square pads (effectively the same as a round pad but with added strength).

You don't have to use the pads that Eagle gives you. I am working on a PCB that is pretty tightly packed and I needed a bit more space, so I created modified DIP packages with smaller round pads. This gave me enough room to squeeze one more track in between the rows of a ROM chip (hope I never have to remove any of the ICs from this board once they are soldered in, because those smaller pads probably won't survive!).