We all know that we use underlay to attach the base fabric to the backing so that the garment will not move within the hoop during the embroidery process, but did you know that there are a lot more reasons, some obvious and some a little more subtle.
They are:
To overcome fabric textures and patterns
To flatten the fabrics surface
To compensate for a loose top density
To create a 3-D effect

Standard underlay
Used to attach base fabric to backing
.1 to .25 in. column stitches use center run stitch
.25 and up add parallel satin
For fill stitches use lattice underlay

To compensate for patterned fabric surfaces
Edge walk adds structure to satins on fabrics with a surface texture such as twill, pique and birds eye knits, this will serve as a base for the top stitching and keep the stitches from following the grain of the fabric and causing a stepped look.

To flatten the nap of a fabric
Terry, corduroy, velvet and any other fabric with a nap require a cross satin underlay to flatten out fibers before you embroider your design to avoid fibers showing through the stitching
A water or heat soluble topping also helps avoid this problem.

To compensate for a loose density
Leather, sheer fabrics, lame and other delicate fabric can not support a normal density without tearing.

To create a 3-D look
To get coverage without raising your density place an inset satin underlay.
Multiple layers of satin stitch underlays can be used to give your embroidery a more 3-dimensional look
Start with a narrow satin and make each subsequent layer a little wider, usually three layers is sufficient.

You are only going to reap the full benefit of underlay if you first understand why we use underlay and which type is appropriate fr your particular situation.

We often talk about embroidery software and which icons do what, but in the end, it’s not all about the software, it’s your abilities that make the real difference!
If you have a real understanding of the basics of embroidery you can really make any software program do what you need. It's what you feel most comfortable using that should determine your final choice.
Let's take a look at what those basics are.

Density
It is very easy to take density for granted. When you power up your software, it will automatically choose a density for you. It could be the default density or a density based upon the fabric that you chose at start up of your new design. This is a reat point of departure but there are so many more things you can accomplish with various different densities.
If you are trying to blend two or more colors together you have to take density into consideration, because when you layer colors over colors the density will quickly build up and the reult will be a thick embroidery which will crinkle on the edges and be very uncomfortable to wear.
If you know what color fabric you are going to embroider upon you can reduce the densities of those areas of your design that are the same color as the base fabric.
If th column widths within your design vary dramatically you would need to increase the densities for the wide columns and reduce the densities of the narrow ones.
Overlapping and Pull Compensation
Using the automatic pull compensation settings of your software can solve many of your overlapping problems, however there are situations in which you will find that you need to make some changes manually. Two of the examples are illustrated above, in both the beachball and the star designs. In the example of the star the last small stitches that would have created the points of the star have been removed and replace with a single running stitch place in a perpendicular position to the satin stitches. If the satin stitches were brought to the end they would have been so small that they either would have caused thread breaks or created a hole in the fabric.
In the case of the beachball, if all of the stripes had met at the top of the ball it would have caused a hole or at the least thread breaks in the last couple of stripes. I exaggerated th overlap to keep the blue stripes from placing needl penetrations in the same place and I kept the yellow stripes from meeting at the top so that they would not cause a hole.