See here or here for the official announcement.
The chief exec at Informa, David Gilbertson, is quoted as saying: "Datamonitor is a model example of a company that slots neatly into Informa...Both companies provide customers with data and analysis that is essential and unique - information they cannot do without. As part of the Informa group we believe Datamonitor will be able to market to our 20 million-strong contact database, take part in our 10,000+ events each year and use our global sales infrastructure across 43 countries. Combining the businesses will enhance growth prospects and margins."
Still no mention of why this acquisition is a good thing for the customer. Perhaps that's taken for granted.