Several reasons:

(1) the Western MBTs involved are seen as a big qualitative improvement over the Ukraine’s (and Russias) Soviet (and in Russia’s case, post-Soviet)–design MBTs.

(2) The release of tanks is seen as a harbinger of loosening on a broad category of heavy Western equipment (manned combat aircraft, for instance.)

(3) The release of heavy vehicles in active production, rather than the prior one-time emptying of stocks of soviety-designed vehicles from mostly ex-Warsaw Pact countries (though some MiG-29s for spares came from the US) is seen as sustainable (and thus something Russia is less likely to think they can overcome by a war of material attrition where they equipment runs out.)

> I can’t see few more tanks in the battlefield doing something to shortening this war.

A qualitative improvement without any quantitative change could, but its not “a few more tanks”, the commitment of Western tanks is something like 15%+ of a generous estimate of Ukraine’s total potential tank inventory (which includes non-operational Soviet-design MBTs from before the way that might be brought up to operational status and captured Russian MBTs), from what I can tell (321 new Western tanks pledged, with somewhere around 2,000 tanks potentially available previously.)