When Russia joined the Council of Europe in 1996, this was seen as a symbol of the democratisation of Russia and of its (new?) concern for Human Rights.
Reversely, leaving this institution today would be interpreted as a huge step backward toward autocracy and a very bad sign about Human Rights in Russia.
Note that while Russia has often protested when ECHR ruled against her, and is fiercely fighting any politics considered as intrusive into its internal affairs, including HR NGOs actions, it is still officially recognizing and enforcing Human Rights.
Adverse ruling by ECHR can be ignored as you pointed out. On the other hand, being part of the Council of Europe is also a way for Russia to occasionnaly point at other (western) european countries failures about Human Rights and embarass them, or to keep an eye on how Russian (or Russian speaking) minorities are dealt with in (eastern) Europe.