Knowing the chromosomal location of a gene can tell you how to design a selection scheme which will result in an improved genotype, it can give you insight into how difficult a task it will be to clone the gene, and it'll tell you whether it's likely that the gene you're interested in shows pleiotropy.
Positional information can be very useful.
Given that we have good maps for most of the crops, it might be reasonable to ask whether we need to resynthesize a map every time we decide to map a gene of interest. It is not absolutely necessary. Consider a gene with two alleles, one with a positive impact on plant height and one negative, in a population of F2 plants. If we perform single seed descent on the population, then eventually the plants will show a bimodal distribution for plant height. If we then bulk DNA from the talls, and bulk the DNAs from the shorts, we should have a uniform distribution of allelic states for all markers unlinked to the plant height gene within each bulk.