Assuming you mean 100mm? Its to do with how near you need to get to the subject your taking pics of to get 1:1
got this from dpreview
Canon EF-S 60mm f2.8 Macro..................9 cm
Canon EF 100mm f2.8 Macro..................15 cm
Canon EF 180mm f3.5L Macro.................25 cm
Canon MP-E 65mm f2.8 1-5x .................10 cm
Sigma 50mm f2.8 EX DG Macro..............4.5 cm
Sigma 70mm f2.8 EX DG Macro...............11 cm
Sigma 105mm f2.8 EX DG Macro.............12 cm
Sigma 150mm f2.8 EX APO Macro...........20 cm
Sigma 180mm f3.5 EX APO Macro...........25 cm
Tamron 90mm f2.8 Di Macro..................10 cm
Tamron 180mm f3.5 Di LD Macro............24 cm
You don't physically have to get so close with a longer focal length. There's not a huge difference but maybe enough not to scare off a bug you're trying to photograph.
EDIT: Beaten by Euan! I'm sure my typings getting slower? That's it Bill... blame the keyboard!
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