Exclusive e-book short stories - are they necessary?

One thing I've noticed this year is that there seems to have been an influx in the release of e-book exclusive short stories ahead of a new book from authors. 

That's not to say that I'm against this as I have purchased most of them but it does seem to disadvantage readers who haven't got an e-reader yet (I would have been included in this category if I hadn't received a Kindle for my 40th birthday). 

Fortunately these e-books either tend to be free or are cheap so it's not going to cost you the earth to buy them.  But is this practice necessary for book promotion or is it simply a means to cash in further?

A selection of these short stories that have been released in the last couple of months or are due out soon are:

I'd be interested to hear from other readers as to whether they buy/read the exclusive e-books before reading the novels? 

Books Read: Alison Lucy - The Summer of Secrets

Source - Received from publisher to review

One heady summer. Three big secrets.

1989. Newlyweds Danny and Harriet arrive at their honeymoon paradise in the Caribbean. Days later, Harriet returns home.

Danny is left distraught but finds comfort in the arms of two women. Nine months later, three baby girls are born…

2010. Megan leaves her childhood sweetheart behind in the UK to go in search of her long-lost father. Miles from home and temptation is at every corner.

Esmé, a Mexican beauty, married Miguel at fifteen. By unlocking the secrets of her past, can she shed the shackles of her enforced marriage?

Claudia has led a life of privilege but she’s never known what it feels like to be loved. Could David be the answer?

Three women set off on an adventure to uncover the secrets surrounding their missing father. It may be the only way to lay their demons to rest but seeking out the truth could tear their lives apart.