As you have seen, every mutation of a gene creates a new alleles. But the significance of these alternative alleles is dependant on the cell type they appear in, namely germline or soma. Mutations that happen in a somatic cell ,for example in the bone marrow or liver, may:

• change the apperance of the cell
• damage the cell
• make the cell cancerous
• kill the cell

Often the mutations have no visible effect at all becasue they don't change the protein like in example number 3. Or they happen in an area of the DNA where there is no gene. The ultimate fate of that somatic mutation is to disappear when the organism in which it occurred dies. In this case the new allele disappears.

However, mutations in a germline cell will be found in every cell descendend from the zygote to which the mutant gamete contributed. That means every cell of the offspring will contain the mutation, including the gametes. So if the owner is able to reproduce, the new allele will be passed on to the next generation and from there to the following and so on. As it was explained in the beginning this new allele might be responsible for a new features like new hair-color of a human or insecticide restitance in insects. In these cases, the phenotypes of the corresponding alleles are distinguishable from the original wild-type allele. But most new alleles that arise in nature do not have a phenotype and therefore have no effect. These alleles are so-called neutral mutations. Interestingly, depending on the environment and the point of time, an allele can be neutral in one situation, but beneficial in another.