Why are box sets so popular?

The Duke Series Boxset One

 When someone suggested I bundle my books - or in other words - put three together and sell them as a virtual boxset, I was intrigued. I put the first three 'duke' books together, Jane Dixon-Smith did the cover and put in a table of contents - and the boxset was ready. I was astonished at the success of this venture - hundreds sold the first week - and suddenly I was earning serious money form my writing.

The Regency Series Boxset One

I think everyone loves a bargain - at £1.99 readers were getting a free book. They are more popular in the US - and my single title sales have dropped in proportion to the sales of the boxsets. As boxsets gain 70% royalty this more than compensates for the loss.

I have told all my friends about this phenomenon and all but one have experienced the same surge of sales. One week in July - the holiday weekend - I sold 1000 boxsets!! Things have settled back and I am now selling around 50 a day - I am also selling around 15  day of my mainstream historical, Barbara's War, Hannah's War and Miss Bennet & Mr Bingley.

 I just love e-books - I buy a several  a week - never paying more than £1.99 - so always know I have something to read. I don't mind if I don't like a book at that price -  I'm prepared to try something new.
However I never buy a boxset - I don't want three books by the same author one go - so can't understand their popularity.
Why do you buy them? Value for money? Or am I missing something here.

This is my latest offering -  published yesterday.