Is it necessary to use flea and worm treatment?

Only if needed.

If you have a flea problem, yes.  Repellantsw may be used to deter them, and a household spray (90% of fleas are not on the dog) with insect growth regulator, and am insecticidal shampoo used to bath the dog rather than stronger chemicals on the dog.

If you don't wish to routinely worm then you will need to do fecal testing, which may be more expensive than worming.

For roundworms it is primarily pregnant bitches and puppies that are most likely to have them.

For adutl dogs that are not in a kennel environment or mixing with above twice yearly worming is sufficient.

Tapeworm is most prevalent in the summer as passed on by fleas.

I am against excessive routine use of poisons on (and worse in) our pets.

Are you at all concerned about lungworm?

I don't use any flea or tick treatments on my dogs, in twelve years of being a dog owner, they've never had fleas. Any ticks I remove myself. I do however worm regularly with Milibax to prevent lungworm, as my younger dog will not leave snails alone.

I worm adults with Drontal twice a year.  I've never seen worms in them though.  I'm not keen on spot ons (which my vet recommends monthly!)  I have insecticidal shampoo in stock (in case I see a flea) but it never seems to get used.  I do have a large canister of Indorex spray here which I use twice a year on carpets, both at home and the caravan. 

So, is it necessary?  Not nearly as much as vets say it is!

I did use to use flea/worm treatments (and vaccinations) randomly, or when told to by vet, as I was always obedient to what they said :B
Over time I came to realise they didn't really need it.  Then I researched a lot more - encouraged and guided by people on here which has led me to a vast array of knowledge and experience.  Some people will try to put you off Facebook groups such as "Does Bravecto kill dogs" poo-poohing it as scaremongering.  I strongly suggest you do your own research and make your own mind up; read up-to-date information, data sheets, VMD, and as much as you can find.  I always find research is more productive if you do one to try to prove the effectivity of something and another the reverse.
You can always check for fleas and ticks with a comb and worms with a poop check with something like Wormcount.com - much better than regularly subjecting your dog/s to pesticides.