Modern pro cameras, lenses and flashguns are awfully costly. A typical digital SLR body can cost between 2,000 and 5,000 pounds. High quality lenses are often one to two thousand pounds each. Flash guns cost a few hundred pounds each. High capacity memory cards can also cost several hundred pounds. Each pro also wants backup hardware in case of mechanical failure and a range of lenses and lights to cope with different conditions. It is kind of ordinary to carry gear around costing between 10 and fifteen thousand pounds.
Regularly this kit is taken out of a studio situation and brought to the client site or to an event locale where members of the general public could damage or thieve it. Accidents happen and though modern pro kit is tough, it's not indestructible.
If a bag of equipment were to be stolen the photographer would replace this at short notice to keep his business running. Individual photo shoots may be covered by renting but this is only a short term solution and would be terribly pricey. To lose 15 thousand pounds worth of kit could be enough to put some photographers into bankruptcy.
Therefore it's a reasonable, responsible plan to have all clobber insured. Make sure that the company you select understand the complexity of the photographic business and cover such things as clobber being left in cars, or at customer locations. Also make sure cover is extended to use by assistants and not only the main photographer.