Does also raise the question as to why they were tried in the first place... Surely a guilty plea at the initial magistrates hearing would have been enough to go straight to sentencing
They didn't plead guilty. A magistrates court cannot try any case where a prison sentence greater than 6 months is a possibility. Criminal damage carries a maximum penalty of 10 years in prison( Section4 https://www.legislation.gov.uk/ukpga/1971/48/section/4) , so can only be tried in the Crown Court.
Whether it's the way you worded it, but that isn't entirely correct. Just because Criminal Damage carries a maximum of sentence of 10 years, doesn't mean cases can only be tried in a Crown Court.
Magistrates Courts
can try and sentence cases of Criminal Damage, depending on the seriousness of the offence. It is classed as a 'triable either way' offence, meaning it can be tried in a Magistrates Court or Crown Court. However, once the evidence is heard if the likely penalty with the facts taken into account, or the defendants previous conviction history, the Magistrates deem they don't have sufficient powers to sentence the defendant, then they will send the case to the Crown Court for sentencing.
The Prosecution too can ask a Magistrates Court that a case of Criminal Damage, after a Guilty Plea, is referred to the Crown Court for sentencing if the circumstances, seriousness of the offence, or the defendants previous convictions, the likely sentence is in excess of 6 months.
But Criminal Damage offences can be heard in a Magistrates Court.