The reason to use depleted uranium shells is to penetrate armored military equipment, thus speeding up elimination of the invading and highly environmentally damaging Russian army.
The environmental damage caused by the Russians in Ukraine far exceeds the damage that could be potentially caused by Ukraine using depleted uranium shells. The Russians caused ecocide in Ukraine, damaging soil, water and air, and releasing a variety of nuclear and chemical contaminants. Uranium shells thus represent some of the most expedient means of saving the Ukrainian environment from being destroyed by the Russians.
References:
After nearly one year, Russia’s invasion has inflicted more than $51 billion in environmental damage on Ukraine, according to the Ukrainian environment ministry.
The invasion, which began on February 24, 2022, has scattered wreckage across roughly 3,500 acres, with rockets and shells scorching some 150,000 acres of forests and plantations, which the ministry said could take decades to recover, even in the most optimistic scenario.
Some 687,000 tons of petrochemicals have burned as a result of shelling, while nearly 1,600 tons of pollutants have leaked into bodies of water. Hazardous chemicals have contaminated around 70 acres of soil. Water and soil pollution could make it temporarily impossible to grow crops in affected areas, the environment ministry said. Complicating matters further, some 15 percent of farmland in Ukraine has been littered with land mines.
Officials are concerned that a recent discharge from the Russian-controlled Kakhovka Reservoir could imperil the adjoining Zaporizhzhia nuclear power plant.
"One Year In, Russia’s War on Ukraine Has Inflicted $51 Billion in Environmental Damage", E360 Digest, Yale School of the Environment, February 22, 2023: https://e360.yale.edu/digest/russia-ukraine-war-environmental-cost-one-year
An explosion due to the shelling of a tank filled with nitric acid in Severodonetsk, 31 May 2022
As of April 1, 2022, more than 36 attacks were registered on fossil fuel infrastructure, 29 attacks on electricity stations, 7 — on water infrastructure, and 6 on nuclear sites. More than 60 fires happened on Ukraine's oil refineries by June 2022. [...] An attack on Lysychansk refinery ignited the 50,000 tonnes tank of oil sludge, two reservoirs with 20,000 tonnes of petroleum, and a sulphur store.
The number of attacks on industrial centres made international observers and Ukrainian government identify them as ecocide. [...] As early as February 24, 2022, the moving of heavy military vehicles raised nuclear dust and resulted in a spike of gamma radiation level in Chernobyl region 28 times higher than normal. [...] According to UN estimation, in 2022 only in Donbass more than 530 ha are considered an area of ecological catastrophe. [...] Ukraine's national parks and reserves are a part of pan-European chain of protected sites titles 'the Emerald Network', they are a home to many endangered species. Preliminary assess showed that more than 1.24 mln (more than a third) ha of protected sites in Ukraine were affected by war.
Environmental impact of the Russian invasion of Ukraine - Wikipedia: https://en.wikipedia.org/wiki/Environmental_impact_of_the_Russian_invasion_of_Ukraine