The Student Room Group
Reply 1
The obvious reason would be getting the benefits/rights provided by the EU (mainly the three communities)-however there is the issue of soveriegnty
Reply 2
Very quickly:

Increase in population:
More consumer - higher demand for products, thus greater price setting flexibility - increase output, economies of scale - higher profit margins for business - increase in GDP. Also, mainly for business: increase in competition for employment - greater demand for jobs - greater wage setting flexibility - higher profit margins. Lower wages/high selling price may encourage industry to relocate to easter europe (esp. with the benifit of HRA, Stability Pact ect), creating labour and generating higher FDI.

More land:
Less competition for land - lower cost of purchase - lower set up costs, encourage non-EU business to relocate. Access to raw materials - lower cost of raw materials - increased profit margins. Lower costs would encourage manufacturing and low-skill labout, especially useful for the ACs.