I used to work for some years as a Traveler Liaison Officer for the local authority so for many years had very close contact with the Traveler and Gypsy Community; both on unauthorized encampments where I carried out welfare enquiries and being an advocate for Settled families on local authority pitches and those who were settled 'in brick' in houses.
I have hesitated to post because I have been in a unique position to see both sides: that of the general public and seeing things from the perspective of Travellers.
Traveller and Gypsy culture is very different to ours and their life is to an extent an anachronism - how we were around a hundred years ago - although the majority of Gypsies and Travellers are now settled and indistinguishable from the general population.
For nomadic Travellers it is hard to understand it from an outsiders point of view; and being a nomadic Traveller is a very hard life - life expectancy is very low, health and diet very poor and education very low. Educationally Traveller children have the lowest school attendance and qualifications and most leave school at 11 to become home schooled (for a boy it means picking up a trade from his father such as scrap metal dealing / paving and for a girl it means taking on a huge amount of cleaning the trailer every day and childcare for other siblings). One stat I read was that only 2% of Travellers live beyond sixty years of age.
There are very strong gendered roles - just as there are in other religions or cultures. Birth control is frowned upon especially for Irish Travellers who are mainly Catholic and very strictly so. Girls must be virgin when they marry and must keep an immaculate reputation and must be isolated from the opposite sex (I knew of one father who turned up at A&E who insisted that his teenage daughter have a certificate to say her hymen was intact because some rumor had been put about that she had been talking to boys and she wouldn't be married as a result). Traveller women may dress what we would view as provocatively but it is a very strict look but no touching policy.
Both girls and boys leave school around 11- not only because traveling and education are very difficult to do but to stop them picking up the immoral values of the settled population and the very real fear of being assimilated into the general population.
Girls are married off very early - around 16- 18 is usual and boys are a little older. A girl does not really have what we would view as a childhood - her whole childhood is preparation for married life- helping her mum with the six hour cleaning of the inside and outside of the caravan every day and cooking and minding the younger children. When a girl is married she is not allowed off a Traveller site unless chaperoned and certainly would be forbidden to speak alone with a man not closely related. The expectation is that she will bear many children - at least five or six and I have known women in their mid thirties with 14 living children.
The pressure of constant cleaning (and I mean keeping living conditions inside the caravan as sparkling), isolation and many children living in a very small space means that women frequently are on antidepressants and are very stressed.
Gypsies and Travelers are subject to a lot of everyday racism, and in turn view gorgers with intolerance and suspicion. In their view gorja women are sluts because we sleep with men outside of wedlock and we are filthy because even a clean house to us is filthy to a Traveller who deep cleans their home every single day. They also view our culture as immoral and are very protective of their children as they view our culture being overrun by paedophiles and child murderers (although tbh in the result of operation yewtree they have a point).
As to the mess left from unauthorized encampments; very few encampments out of the hundreds I dealt with left nothing behind. There was usually litter, rubbish and human waste but it would be in proportion to the size of the encampment - large encampments of over 60 caravans would leave trade waste etc and have heavy human soiling and cost around £6000 to clear while small encampments a few hundred pounds. I understand that this caused immense annoyance to the local population and I was always asked why they did this.
To Travellers the outside is not as important as the inside living quarters. No Traveller would have an inside chemical toilet as they view this as abhorrent and disgustingly germy. To them it was far healthier to go to the toilet in the open as nature intended - and they would go usually in the footpaths and bushes surrounding encampments discreetly.
As to provision for Traveller sites; there used to be a legal duty to provide Traveller sites by local authorities which was taken away. Now there are around 6000 fewer pitches than nomadic Travellers which means there are few places they can stay legally. Travellers would never go to tourist caravan parks (and tbh I can see few tourist parks who would accept them due to perceived antisocial behavior), so the problem of unauthorized encampments is likely to continue.
As to why they they have to travel - both Gypsies and Travellers have a very long history of travelling (over a thousand years for Gypsies and two thousand years for Irish Travellers) it is their culture and they feel the same way about being forced into brick as we would feel being forced out of our houses to take on a nomadic life.