It's simple: the winners of the 2018/19 UEFA Europa League have the added incentive of gaining automatic entry into the 2019/20 UEFA Champions League group stage.
Has that always been the case?
When the rule was introduced for 2014/15 (for access for 2015/16), winners were initially assured only of a UEFA Champions League play-off place, going straight into the group stage if either themselves or the UEFA Champions League holders had already qualified via their domestic league. That, in fact, always happened in the three seasons that particular rule existed. It switched to automatic entry last term.
What if the winners have already qualified via their domestic league?
If the UEFA Europa League winners qualify for the group stage via their domestic league, the third-placed team of the fifth-ranked association (France) enter the group stage. This happened last season: UEFA Europa League winners Atlético Madrid qualified via their domestic league so Lyon, third in France, entered in the group stage.
Could a side in an 'automatic place' in their domestic league miss out?
It hasn't happened yet but in theory, yes. National associations can have a maximum of five clubs in the UEFA Champions League so it is possible, but very unlikely. The following would need to happen:
> The UEFA Champions League winners and UEFA Europa League winners would have to be from the same nation (namely England this season*);
> They would both have to finish outside the top four in their domestic league.
If both conditions were met then the team finishing fourth in that domestic league would instead go into the UEFA Europa League group stage.
*Germany, Italy and Spain were also possibilities but the Bundesliga has no representatives in the UEFA Champions League quarter-finals while the sole UEFA Champions League hopefuls from Italy and Spain, Juventus and Barcelona, are already certain of top-four spots. As are Liverpool in England.
Who has benefited so far?
2014/15 – winners Sevilla (finished fifth in Liga)
2015/16 – winners Sevilla (seventh in Liga)
2016/17 – winners Manchester United (sixth in Premier League)
2017/18 – Lyon