Of course most people do plead guilty. But for the few that don't, the jury still matters. For example, the meaning of the word indecent is not defined anywhere in the law. A judge may take a different view of what that word means, compared to a jury. 

There have been a handful of cases where the CPS have prosecuted innocent images of children, taken in the context of family photos, for being indecent. There have also been cases of explicit cartoons being sent as a joke, that have gone to court.

In past history, there is a pattern of prosecutions under the old obscenity laws, where porn magazines were prosecuted for being obscene too. But when the jury was shown the images in court, they rejected the argument that it was obscene.

On the whole, the public is more broad minded than either the police, or the judiciary.

If you're guilty, plead that way, no jury needed then. 

The latest proposal to abolish jury trials for likely sentences of under 3 years, seems to create some confusion, because what does likely actually mean?
There are many offences for which the sentencing range is between 2 and 5 years. Distribution of Cat A images for example.

The starting point in the sentencing guidelines is 3 years. The maximum sentence is 5 years, but the sentencing range is from 2 years to 5 years. So what is the likely sentence? In practice, offences of this kind tend to receive a sentence of 2 years suspended in most cases. Statistically then, the most likely sentence is one of 2 years, despite the starting point being 3 years. So could these offences still go to a jury?

Arguably, all sexual offences should go to a jury, because the consequences of being found guilty are so serious, even for a possession charge. The impact, thanks to the Google effect, is potentially lifelong. 

Then there are the other consequences, such as a potential 10 year SHPO. That is not even being taken into account, in deciding whether to allow someone a jury trial.