I might agree if Spain was just a "pointless friendly". It's not, it's part of an international competition leading up to the Euro's. It also has trophies as a recognised competition. England and Southgate needs his best players available.
https://www.channelnewsasia.com/news/sport/football-the-uefa-nations-league-explained-10681888
The UEFA Nations League tournament sees the 55 European national teams split into four divisions, taking promotion and relegation to international football while also offering an alternative route to the 2020 European Championship, without replacing the qualifiers themselves.
UEFA are using their own coefficient to split their 55 member nations into four leagues, with the top 12 sides all in League A. The next 12 are in League B, followed by 15 in League C and the bottom 16 in League D. Each group contains three or four teams, with everyone playing each other home and away between September and November this year.
The winners of the four groups in League A will go through to the finals from Jun 5-9 next year. Semi-finals, a third-place play-off and a final will all be played, with the host country to be decided in December from among the finalist teams. There is a title and a trophy up for grabs, but also promotion and relegation with future Nations Leagues in mind. The team finishing bottom of each group in Leagues A, B and C will be relegated, and replaced by those finishing top of the groups in Leagues B, C and D.
There will still be qualifiers for the 24-team Euro 2020. They will take place from March to November, 2019, with a draw to be held this coming December. However, that will no longer be the only means of qualifying for that competition. Twenty teams will make it to Euro 2020 from qualifying, with the top two in each of the 10 groups advancing. The remaining four places will be decided in March 2020, in the Nations League play-offs.