Synapsis is the pairing of homologous chromosomes before segregation. This period permits crossover. While autosomes undergo synapsis during meiosis, sex chromosomes often remain unpaired.
An array of proteins assembles between the two homologs. The non-sister chromatids in the tetrad align together at any point they find to match then zip together into a synaptonemal complex.
The synaptonemal complex proteins binds the homologous paired chromosomes together during prophase I of meiosis and promotes genetic recombination by the recombination enzymes that complete the crossover. This aligns the two at paired gene loci despite having different alleles for the genes.
This process of synapsis aligns the two homologous chromatids at gene loci despite having different alleles for the genes. This even aligns segments despite tandem repeat length differences
Recombination increases genetic variability within both offspring and the wider population by making new allele combinations possible despite being on the same chromosome. Crossover also provides a mechanism for repairing a damaged strand of DNA by comparison with the homologous, but intact, strand. This is a way to keep more integrity or fidelity in larger genomes.