There are benefits to keeping worms in a garden. Alongside microbes, they break down dead organic material to make nutrients accessible for plants.

But this is not the case all the time.

If there is nothing for them to consume, worms may turn towards plant roots. A case where this happens is when worms are confined in a limited space such as in a pot.

It is therefore important to keep worms fed by adding organic materials like dried leaves, plant snippings, grass clippings, or compost mix to the soil. 

Another important thing to know is that not all worms or worm-like organisms help provide nutrients in the garden. Others are considered pests as they just eat plant matter.

Examples include larvae of moths, beetles, and butterflies. Larvae have a large appetite for plant matter as they need to store plenty of nutrients when it turns to a pupa. 

Also be on the lookout for plants suffering from stunted growth as they may be infected with microscopic nematodes. Examples of such nematodes include the root-knot nematode, root lesion nematode, and dagger nematode. It  is worth noting, however, that the majority of nematode species are not really harmful.

Should all of these things be considered, worms can help a garden in various ways.

They tunnel through the soil, thus spreading out nutrients from dead organic matter. 

More than that, worms play a great role in biodiversity. The nutrients worms spread out in the soil are also consumed by microbes, which in turn also contributes to plant growth. Aboveground, birds prey on worms. Having birds in your garden is an advantage as they also prey on garden pests.

Worms also help in soil aeration. The underground air pockets worms leave on their trail help retain soil moisture thus keeping the soil cool.

Source

Source