Crossover causes a recombination of the genes. This combined with other factors, such as mutations, are important if you want an abundant amount of biologic variability. Without crossover, there would be too many linked alleles that would be passed on to any offspring, which would create a very large lack of variability and the species would die out. If the parental gametes do not contain the best genes that are needed for the species to carry on, and without crossover, those genes would be passed on and on, leading to eventual extinction.
Genetic variations in the future generations would be restricted. If a bad gene is linked to a good gene, the good gene has the same level of risk as the bad gene of being removed from the population.