Most of the focus on the Collective Security Treaty Organization has been its Central Asian activities, as Russia has positioned the new political-military bloc as its primary tool for preventing the spread of instability from Afghanistan toward its borders. But as Sergei Minasyan points out in a good piece for Russia in Global Affairs, it is in fact Armenia for whom the CSTO really holds strategic value. As he points out, among CSTO members (which include Russia, Armenia, Belarus, Kazakhstan, Kyrgyzstan and Tajikistan) only Armenia faces a threat of interstate conflict. (One might quibble with that, looking at increasing tensions between Uzbekistan and its neighbors Kyrgyzstan and Tajikistan, but certainly the threat of serious military conflict is much smaller there than between Azerbaijan and Armenia.) And the collective security requirements of the CSTO effectively make it impossible for Azerbaijan, in the event that it decides to try to take back its breakaway territory of Nagorno Karabakh, to widen the conflict into Armenia. And that would allow Armenia, if Azerbaijan attacks Karabakh, to use the latter's territory for missile strikes against oil and infrastructure facilities while remaining "unpunishable":

As a result, Azerbaijan is in a military and political zugzwang, which effectively prevents a resumption of war. A direct involvement of the CSTO (or even Russia alone) would make the likely outcome of combat operations in Nagorno-Karabakh more than predictable. Starting a war in Karabakh without spreading it to the territory of the Republic of Armenia (so as to provide no reasons for the CSTO mechanisms and bilateral Armenian-Russian obligations taking effect) would contradict military logic and put Baku in disadvantageous military strategic conditions.

As a result, Armenia is probably the most active participant in the CSTO (other than Russia), placing particular importance on development of the Collective Rapid Response Force (CRRF):

Yerevan believes that the most urgent measures must be taken to develop the CRRF’s potential. The specific nature of current regional threats in the South Caucasus and the fast-moving nature of modern combat require fast intervention and response measures. For instance, to make the CRRF more mobile it is feasible to store the necessary amounts of armaments and other military hardware in the territories of the participating countries, which would make it possible to airlift personnel armed only with light firearms within the tightest deadlines. The heavy armaments and equipment would be already in place and readily available.

Yet, the CSTO here is in fact only Russia. While nominally an alliance, the group's Central Asian members will likely have little incentive (and in most cases, capability) of intervening in an Armenia-Azerbaijan war:

Yerevan has no illusion that, should a force-majeure situation emerge in the South Caucasus, Kazakhstan would order its naval ships to drop anchor off Baku, and/or Kyrgyzstan would dispatch mechanized infantry to the mountains of Karabakh. Yerevan would have shown greater interest in the problems of security in Central Asia (the Armenian army’s rapid response force and peace-keeping contingent are trained well enough to do that) if it were certain that its Central Asian allies would take symmetrical and proportionate actions in the Karabakh conflict. Therefore Yerevan has to consider the possibility of the CSTO’s assistance in addressing regional security problems in the South Caucasus mostly in the bilateral Armenian-Russian format.

This was illustrated during the CSTO's last big exercises, which took place in Armenia but during which any discussion of Azerbaijan or Karabakh was very muted.

However, Minasyan does note that Uzbekistan's departure from the CSTO will make it easier to gain political support for Armenia against Azerbaijan:

It is common knowledge that Tashkent had in fact torpedoed many CSTO initiatives, specifically those concerning crisis settlement measures, which at a certain point forced establishing – largely at the initiative of Moscow and Yerevan – a decision-making mechanism to use CSTO structures in the absence of a consensus. Also, Uzbekistan’s attitude to the Karabakh conflict based on the idea of “Turkic solidarity” was dissonant (to put it mildly) with the position of Armenia – its military and political ally to which Tashkent had certain formal mutual liabilities in the sphere of defense and security.