The perception is that by negotiating trade deals individually, the deal can be tailored to the individual country. (politics.stackexchange.com)
The EU has also helped cement peace in Europe since the Second World War and gives members more weight when negotiating trade deals with other major trading blocs such as North America and China. (mirror.co.uk)
One thing that the EU had been accused of doing prior to Brexit, is favoring the continental countries when negotiating trade deals, sometimes at the expense of UK interests. (politics.stackexchange.com)