EU Member states can choose whether they want to have multiple constituencies for elections to the European Parliament, or not. Many EU member states have a single national constituency (like Germany) while others have subdivisions (like Poland, and formerly France). What is the point in the choice? And why do some nations choose a single constituency and others multiple?
Member states are allowed to elect its MEPs in any way they wish, so long as it produces a proportional result.
With respect to the choice with regards to constituencies, there are pros and cons to each of these approaches.
The primary advantage to having a single nation-wide constituency is that it provides the most proportional result. Many smaller countries don't have enough seats to be able to divide into constituencies while still maintaining a proportional result.
The primary advantage to having multiple constituencies is that it actively prevents a capital-centric selection of MEPs, and ensures that every part of the country has representation in the European Parliament.
European countries have different traditions and laws for their national elections. European elections are close enough to a common standard to satisfy most requirements of democracy and close enough to the national traditions to feel familiar and comfortable to the voters.
For instance, it is generally accepted that elections should be on the same day for the entire electorate, so that no voter can use partial results for tactical voting. European elections are on different days, but results are only communicated at the last day.