Fleeces are highly versatile and available in a wide range of thicknesses making them a great choice as a mid-layer. Thinner fleeces are very practical, quick drying and breathable, making them a good option for high-output activities such as hiking in cold weather. Synthetic fleece material is also hydrophobic, which means it repels water and dries really fast, and it’s fluffy construction helps to trap air and retain heat from the body. In cold conditions where you are standing still or inactive, a thicker mid-to-heavy weight fleece will be better suited to make sure you keep sufficiently warm.
Although fleeces have great insulating qualities, on the downside it’s worth pointing out that fleeces can weigh more than other forms of mid-layer insulation and aren't as compressible so can take up more room in your pack. Fleeces are also prone to pilling (when the fibres start clumping together), which makes it less insulating over time, and doesn't look so great, either.