Districts are good to make sure there are some local ties between citizens and representatives and not only represantitives from one region remain.

Take the German system: there are district level elections, so that there is one person elected in each electoral district. These get half the nominal seats. Then the other half of parliament is filled up with candidates to match the proportional vote. The fill up is a bit complicated, especially if a party has a lot more directly elected folks than proportionally mandated, which leads to growth of parliament and discussions how that growth can be avoided. (Larger districts and only 1/3 directly elected or not every directly elected candidate really making it or not holding up the exact proportionality or whatever) But it combines the factors mostly fine and avoids many of the problems of the pure systems.