It's an old school chemistry safety tip. Adding the water to the lye could cause the lye to splash out and the concentration will be very strong at that point. So get your sturdy container, add water, then mix in the lye very carefully. Lye is no joke, so be very careful.
Some people will keep separate lye only measures, bowl, and containers to prevent the chance of cross contamination. Google lye safety and I'm sure you will find plenty of solid tips.
And keep a large bottle of vinegar handy with the lid off in case of accidents.
This is based around soap making, but the same rules apply.
http://www.soap-making-resource.com/soap-safety.html