The italian tyre supplier is facing hard work ahead: a different rubber is requested by the teams for the 2020 season and technical regulation changes regarding the suspension/tyre package will require a brand new concept for 2021.
Pirelli has therefore asked the teams, on a voluntary base, to take part in some tests to improve the final result. This apparently will not play as a key development role for the teams since they won't be told what rubber they will be running on.
Still, constructors have decided to run, with Ferrari joining Mercedes and Red Bull in Montmelo. The difference between those teams is going to be the test driver: while Mercedes will run reserve driver Esteban Ocon and Red Bull have chosen their simulator driver Jake Dennis, the Scuderia will send Sebastian Vettel himself.
The German will then have to fly straight to Japan for the race in Suzuka.

If the driver transfer is a problem just for the Maranello's team, all the three of them will have to face another big challenge: logistics. All the mechanics and cars' components will have few time to reach the asian country, with no room for delays.
“We’re heading to Suzuka and clearly with Honda we want to be in the best shape we can be and now we are being pulled to Barcelona as well, so it’s mighty challenging" said Red Bull Chief Engineer Paul Monaghan.

This test was indeed squeezed into a busy calendar at the last moment possible, on Pirelli's request: “Testing is very important for us. We need to validate our tyres on track. We cannot change the product during the year. We have to do the best we can do during the season in order to homologate a good product for the following season and the only opportunity is to have a proper test calendar” stated Mario Isola.
The aim of the test is to have another comparison on the 2020 rubber, after the last test at Paul Ricard: “We made a proposal to have an extra test to test a new compound with a wider working range, especially on the hard [tyre] levels."

https://www.motorlat.com/notas/f1/13161/kimi-raikkonen-on-his-losing-streak

Ferrari’s Sporting Director Laurent Mekies explains why it's crucial to attend the run: "We felt it was a duty towards everything we are asking to Pirelli to support the fact that we all want better tyres and that was the best way to achieve that.”
James Allison, Mercedes’ Technical Director, shares the same opinion: “It’s tough, but it’s doable. The aim of getting better tyres is a noble one. We’re just leaning heavily on people who are knackered but they’ll step up…”
“We are grateful to the teams that accepted to test for us. I fully understand that it is a big effort of them to fit this tyre test into the calendar, which is very, very busy. It will be the same next year with an additional race, so we are finalising also the plan for next year” Mario Isola concluded.