While the facts in Comstock are about at least one very bad person who will be diverted to a civil commitment facility, the importance of the case extends well beyond the fate of Graydon Earl Comstock, Jr. Among the constituencies and issues that might be affected, there are at least five potentially important results that could come out of the SCOTUS opinion.
First, as the first post-Raich case that might concern Commerce Clause doctrine, Comstock is obviously notable. If the Court retreats from the jurisprudence led by Justice Rehnquist in Lopez and Morrison, then it is quite likely the Commerce Clause will revert to being a meaningless bar to federal jurisdiction. In the alternative, the Court might continue its ideological pattern with Commerce Clause cases wherein liberal statutes (gun control and the Violence Against Women Act) get struck down while more conservative laws (drug laws and sex offender civil commitment) are upheld.
Second, if the Court wants to avoid meddling in the messy area of the Commerce Clause, it might pin a decision for the government on an expansive view of the Necessary and Proper Clause (as the government brief has asked the Court to do). Given the limited role of the Clause in recent decades, a reinvigoration of it could have even greater effects in expanding federal jurisdiction than a purely Commerce Clause opinion. Because the Necessary and Proper Clause applies to any of the enumerated powers, a Comstock opinion based upon it could end up super-charging claims of federal jurisdiction in a variety of contexts.
Third, if the Court decides for Comstock, the rest of the AWA might be reexamined under the Commerce Clause. As I have argued elsewhere, the Commerce Clause basis for SORNA is tenuous (although stronger than that of the civil commitment provisions at issue in Comstock). While a large number of district courts dismissed SORNA prosecutions on the argument that the Commerce Clause could not support such prosecutions, every appellate court so far has gone the other way. A defendant victory in Comstock would reopen the federal jurisdiction issues in such cases.
Fourth, if the government wins, the due process issues that were a second basis for the district court opinion will still need to be resolved. The government has sought a much lower burden of proof for civil commitment than has been tested before. If that lower burden is ultimately upheld, it might effectively end sex offender treatment in prisons (because statements in treatment by offenders are used for commitment proceedings and virtually assure commitment). Further, the burden of proof holding could extend to other areas of non-criminal commitment and preventive detention.
Fifth, if Comstock loses regardless of jurisdictional basis, there is a genuine risk of some ugly bootstrapping scenarios using the civil commitment provisions or similar laws. Consider a situation where government wants to incarcerate someone, has evidence for an arrest (or depending upon how "custody" is interpreted, merely an interrogation), but not nearly enough for a prosecution and conviction. Or there might be an instance where there is evidence for a petty crime, but not for the higher crime that the government wants to prosecute. In either scenario, the government could bring someone into custody and then divert them to civil commitment with less procedural protections and a far lower burden of proof. Further, the diversion could end up lasting for a lifetime far in excess of the potential duration of criminal penalties. It really would allow an end-run around the normal due process protections in place during criminal arrests and trials.