explain rationale of start-tor-browser.desktop using --deteach

Could you explain please the rationale of using --deteach?

(Which was decided in legacy/trac#15741 (moved).)

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(I could not reply in legacy/trac#15741 (moved). [Warning: The ticket field 'parent' is invalid: The parent ticket (#15747) is not an active ticket (status: closed).] And I thought removing or reopening the parent would not be appreciated.)