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Because simply favoring an existing trait vs. coming up with something that never existed before are two completely different things.

Yeah. But the difference between the former and the later doesn't make studying the former illegitimate or uninteresting. Besides, we already know that new traits can arise by evolution. E.g. (links swiped from Ichneumon here, see also the next post):

Directed evolution of biosynthetic pathways. Recruitment of cysteine thioethers for constructing the cell wall of Escherichia coli
Directed evolution of a type I antifreeze protein expressed in Escherichia coli with sodium chloride as selective pressure and its effect on antifreeze tolerance
Molecular evolution of an arsenate detoxification pathway by DNA shuffling
Long-term experimental evolution in Escherichia coli. XIII. Phylogenetic history of a balanced polymorphism
Rates of DNA sequence evolution in experimental populations of Escherichia coli during 20,000 generations
The evolutionary origin of complex features
Contribution of individual random mutations to genotype-by-environment interactions in Escherichia coli
Rapid phenotypic change and diversification of a soil bacterium during 1000 generations of experimental evolution
Bacterial evolution and the cost of antibiotic resistance
The ecology and genetics of fitness in Chlamydomonas. IX. The rate of accumulation of variation of fitness under selection.
Mild environmental stress elicits mutations affecting fitness in Chlamydomonas
The emergence and maintenance of diversity: insights from experimental bacterial populations
Direct Estimate of the Mutation Rate and the Distribution of Fitness Effects in the Yeast Saccharomyces cerevisiae
Pleiotropic effects of beneficial mutations in Escherichia coli
The Rate of Compensatory Mutation in the DNA Bacteriophage X174
Mutation-selection balance accounting for genetic variation for viability in Drosophila melanogaster as deduced from an inbreeding and artificial selection experiment
Genetic restriction of HIV-1 infection and progression to AIDS by a deletion allele of the CKR5 structural gene
Complete Rescue of Lipoprotein Lipase–Deficient Mice by Somatic Gene Transfer of the Naturally Occurring LPL^S447X Beneficial Mutation
Evolution and Information: The Nylon Bug
Spontaneous mutations in diploid Saccharomyces cerevisiae: more beneficial than expected
Nonuniform concerted evolution and chloroplast capture: heterogeneity of observed introgression patterns in three molecular data partition phylogenies of Asian Mitella (saxifragaceae)
Evolutionary analysis of genetic variation observed in citrus tristeza virus (CTV) after host passage
Examples of Beneficial Mutations and Natural Selection
Genetic Variant Showing a Positive Interaction With ß-Blocking Agents With a Beneficial Influence on Lipoprotein Lipase Activity, HDL Cholesterol, and Triglyceride Levels in Coronary Artery Disease Patients
Genetic restriction of HIV-1 infection and progression to AIDS by a deletion allele of the CKR5 structural gene
Evolution of new information
Spontaneous mutations in diploid Saccharomyces cerevisiae: more beneficial than expected
Are Mutations Harmful?
Evolution and Information: The Nylon Bug
Directed evolution of human estrogen receptor variants with significantly enhanced androgen specificity and affinity
Multiple duplications of yeast hexose transport genes in response to selection in a glucose-limited environment
Complete Rescue of Lipoprotein Lipase–Deficient Mice by Somatic Gene Transfer of the Naturally Occurring LPL^S447X Beneficial Mutation
Punctuated evolution caused by selection of rare beneficial mutations.

PLEIOTROPIC EFFECTS OF BENEFICIAL MUTATIONS IN ESCHERICHIA COLI
The Distribution of Fitness Effects Among Beneficial Mutations