Titanium grid fins is a big reason they recovered the 2 side boosters. This launch and landing will be used to fine tune their simulation parameters.

The block 5 variant is designed for 10 launches in a row with 24hr turnaround. With service, over 100 launches is the goal.

Wow! that's impressive!

SpaceX has a fairly large collection of graphics cards for simulation purposes see https://www.youtube.com/watch?v=vYA0f6R5KAI and https://www.youtube.com/watch?v=xNqs_S-zEBY

The current version of the rocket is called "block 5". Those two boosters weren't block 5 boosters. Presumably the reason to spend resources to get them to safe landing was to learn things that will be useful in future launches; for example, those two boosters had enough ignition fluid to land successfully this time, but the main first stage did not.

Precisely. "We can afford to lose these ones on a test flight."

That make sense. In that case, wouldn't these boosters be useful in future Falcon Heavy launches? Why can't they be resued?

At this point they're historic artifacts worthy of preservation. Retire them to a museum.

During the news conference after the launch, Elon commented on being really happy to get the upgraded titanium grid fins back from the side boosters:

https://youtu.be/F7mw2_pfcz4?t=3554

These are the first two boosters used to launch a Falcon Heavy. They are modified Falcon 9's. They already know much of the wear and tear that Falcon 9's endure in flight from previous launches. They didn't yet know how their boosters are affected by the modifications required, and the stresses involved in launching a Heavy. Now they have two pristine pieces of data they can tear down and analyse for potential improvements.

Training.

That, and the best equipment to test out is equipment you know you won't need again in the future.

Case in point: It's much cheaper and less disruptive for them to lose this rocket and fix the issue than to lose a future rocket slated for reuse.

Demonstrating that the system can work as intended is the point of the test, and that includes recovery. The boosters in particular have different aero. Even though these won't be reused, they serve to demonstrate that future vehicles can be.

Probably worth having done just for that beautiful video of the two of them landing together... You want credibility in the marketplace? You just can't buy the things that imagery says about your business/capability.

They are older versions of the boosters, the new Block 5 versions are designed to be re-used many more times.

> Why did SpaceX spend resources to get them to safe landing then?

collect data + parts of the booster can be reused

Recovering the boosters on land is practically free. Landing at sea is less so, but they want to ensure they can recover the center booster in the future, so they probably want to start testing right away so they can find problems like this one before they start losing boosters they actually want to reuse.

They might have plans for them, like they did with this booster: https://en.wikipedia.org/wiki/Falcon_9_booster_B1019

To analyze/test the used boosters, or to put them on display, probably

Probably both, particularly with the main booster now lost.

"This was the pair of boosters used on the historic first flight of X to Y" looks good on a museum plaque.

Scrap