One of the tricky parts of finishing pasta with a butter sauce is the sauce can separate or get too thin. The emulsion (mix of fat and liquid) doesn't hold. You can avoid this by using cold butter when creating your sauce.
Photo by K. B. R.
Chef Mario Batali shares this tip among his list of the ten most important pasta cooking tips. The reason for the cold temperature: heat (as in warm pasta) can cause the delicate balance between fat and liquid in the emulsion to break. If you use cold butter, you have less chance of the sauce becoming too hot and you'll get a better emulsion.
This is true for other emulsion sauces as well, Fine Cooking points out, such as hollandaise sauce.
The 10 Most Important Pasta Cooking Tips [Mario Batali]