Why buy a sampler? Think of it as getting a whole tool kit, instead of just a hammer. White Sage is an excellent all-around choice for smudging, and it is particularly good at dispelling negative energies. Cedar is also good at eliminating negative energy, but it has the added benefit of attracting good spirits into your home. Yerba Santa, on the other hand, is best known for protection, setting boundaries, love, purification, and growth.
And best of all- you'll save money by buying the sampler instead of purchasing each herb individually.
How do I know which bundle to use? You can gain some insight by looking at the properties of each herb. But in addition, use your intuition to guide you. As you gain more experience with smudging, you'll find that you have an affinity for certain herbs, while others may be less effective for you. Experiment! Try different things and see what works for you. There are no wrong answers, and you aren't going to cause any harm by using one herb bundle over another. Over time, you'll learn so much, and be able to refine your practices.
Can I burn more than one bundle at a time? Certainly! Blending the smoke from two different herb bundles changes the dynamics a bit. You'll find that the plants' energies may be expressed differently. Again, experimentation and experience will stand you in good stead.