Crimea? Any fool with a map could see that it's a useless white elephant without a secure water supply which only Ukraine can deliver, so...
Crimean oblast was transferred from Russian Federation to Ukrainian SSR by Khruschev in 1954. And it was made with violation of Constitution (approved by Chairman Board of Supreme Council while by Constitution any territorial changes must get approval of whole Supreme Council). There was post-factum approval of Supreme Council that still leave whole thing questionable.
As for City of Sevastopol - it was never transferred to Ukrainian SSR and was a city with direct subordination to Union (i.e. on same scale as Ukrainian SSR itself). It was "forgotten" once Ukraine get independency and local nationalists started to grab everything they can (so later they begging that they need foreign financial help for safe utilisation of huge amounts of outdated artillery shells in a army warehouses they quickly declared property of Ukraine). Status of Sevastopol was questionable until 2014.
If you are interested which referendums was in Crimea in beginning of 1990's and how their results was finally ignored by ukrainian nationalistic politicians - you have to google little bit. But referendum of 2014 sorted it out.
Concerning usefulness - Sevastopol is the only really good navy base on this side of Black Sea, and key to defense of Southern Russia from any military threats from Black Sea..
It was under foreign military occupation when the 'referendum' was carried out...
There was personnel of russian navy base that was stationed there on a legal basis. And not a single round was shot in process.
Eastern Ukraine? Essentially useless land with the least social capital in the country?
Eastern Ukraine, AKA Novorossia is again Russian land with predominantly russian population that was de-facto secessed after german occupation and transferred to a newly created ukrainian republic. Also, far from "historical ukrainian lands".