There is a limited amount of water and nutrients in any garden. Since plants get their nutrition from the soil, weeds are also working to obtain some of the limited amount of nutrients for themselves. When they do, it reduces the amount of available nutrients for your plants. The same things happens with the rain in your garden. Especially if you live in an area with limited rainfall, plants will struggle to get enough water if weeds grow in a garden not designed to support them.
The more weeds you have, the more space in your garden is monopolized by plants you didn't choose. If they're taller than the plants you've planted, your plants could miss out on nourishing sunlight when the weeds throw them into shadow. This is a bigger danger for smaller plants or seedlings than for larger plants like sunflowers or corn.
An influx of weeds can lead to other potentially harmful problems -- primarily diseases and insects. Even if you've planted insect-resistant flowers, your weeds will harbor insects that will chew their way through any other crops you have. Weeds in a garden also allow extra opportunities for maladies to catch on and rampage through your plants. According to the website Gardening Info Zone, your plants are more likely to get fungal diseases when they're crowded by weeds.
Get rid of weeds as soon as you spot them. Once they mature, they drop seeds and allow even more weeds to grow. That's why a few weeds can quickly spiral out of control and take over your garden. If you have a small garden, you can take care of the problem yourself with a little time, a spade and a compost bag. A larger garden may require stricter measures, like a tiller, plastic overlay or mulches.