Some people buy lenses because they can, others buy them because they need them. Then there are the people who straddle both camps...
Effectively, you've got from 18mm through to 270mm covered in one lens, which is a very useful range for general shooting.
The key factor in deciding whether or not you need more lenses and how much you spend, are the gains associated with upgrading.
Take your 18-270mm for example; it'll be something like f/5 at the wide end, changing to f/6.7 at 260mm. Ideal for good light where decent shutter speeds will still be available (without cranking up the ISO) but get to low light situations then you either need to increase the ISO to cope, and/or invest in a faster aperture lens. You could get something like an f/2.8 120-300mm Sigma, a superb lens, but it's going to be the best part of £1600 new/£1000 used but you have a loss of overall focal length - you've gained 30mm at the long end - but you've gained several stops of light gathering power. It's cost a lot of money to get those gains though....
It's thinking like this that usually leads to people buying new lenses.
My own path was like this; I had a 18-70mm Nikon that was f/3.5-4.5 but I wanted a constant f/2.8 lens so swapped for a Tamron 17-50mm. I couldn't afford the Nikon 17-55mm at the time so this was the perfect option. Once I got the funds together, I bought the more expensive Nikon version. Effectively, I've gained pro-spec build quality and optical perfection at f/2.8, although in effect I've lost 25mm of coverage at the long end when compared to that cheaper Nikon kit lens. But the gains are worth it.... then I went mad and just bought loads of different lenses
I shoot professionally but I've concluded that aside from a long, fast prime (eg: 300mm f/4) I have everything covered in five lenses - 11-16mm f/2.8 for the wide editorial stuff; 17-55mm as my standard 'walkabout' lens; 35mm f/1.8 for low light and as a general video lens; 60mm macro for close-up work; 70-200mm as my long lens option, combined with a teleconverter if required. I've had all sorts of other lenses and there are lenses that appeal to me (14-24mm f/2.8 for example) but I know they're luxurious rather than essentials. All my lenses are the fastest available and that matters a lot, seeing as I work outdoors and need as much light gather ability as possible.