Usually monofilament wigs are made from real human hair. A monofilament cap takes time to make, so there is no point in making top quality wigs from synthetic materials. Besides, synthetic fibers are not as durable as natural human hair.
Most modern lace wigs marked "monofilament wig" are actually only partially monofilament. Usually the most important part of the wig, the top, is monofilament, while the back has a more traditional construction. This reduces the cost of the wig and makes it easy to adjust with tape tabs. These ensure that your wig fits snugly on your head and will not slide.
Many wig designers offer monofilament full lace wigs. Wigs by Rene of Paris and Louis Ferre, in my opinion, are especially comfortable and natural looking. Unfortunately, monofilament wigs are never cheap - they take a lot of time to make. But on the bright side, hand-knotting makes the wig more durable. So with good care, your wig will serve you for several years - even if you wear it every day.
A monofilament wig cap offers a lot of benefits, but they will mostly be appreciated by people with severe hair loss. If you wear a wig just for fun over your own hair, a transparent cap makes no difference.