Nutshell: If nitrate is given without pauses interval, nitrates will not be as effective => need to pause in between nitrate administration (Wonder Me!) 

Nitrates exhibit development of tolerance, or more specifically tachyphylaxis, meaning that repeated application results in a fast decrease of effect, usually within 24 hours. A pause of six to eight hours restores the original effectiveness.
Mechanism:
-  A pause of six to eight hours restores the original effectiveness. This phenomenon was originally thought to be a consequence of depletion of thiol (–SH) groups necessary for the reduction of nitrates. While this theory would fit the fact that molsidomine (which is not reduced) does not exhibit tachyphylaxis, it has meanwhile been refuted.
-  Newer theories include increase of oxidative stress resulting in deactivation of NO to peroxynitrite, and liberation of the vasoconstrictors angiotensin II and endothelin as the blood vessels' reaction to NO-mediated vasodilation.
 (Source: https://en.wikipedia.org/wiki/Nitrovasodilator#Mechanism_of_action)

nitrate-free interval 2 prevent tolerance

Despite undeniable benefit in relieving acute angina, the clinical use of organic nitrates is complicated by the development of tolerance to their pharmacologic effects. Nitrate tolerance is characterized by loss of hemodynamic and antianginal effects after repeated dosing; higher and higher doses are needed to obtain the same physiologic effect. Tolerance to intravenous, topical, and transdermal nitrates can develop in as little as 24 hours. The use of the long-acting oral nitrates, isosorbide dinitrate and isosorbide mononitrate, also results in clinically significant nitrate tolerance in as little as 1 week, depending on the dosing schedule used
In clinical research, tolerance to nitrates typically manifests as a significantly decreased time to angina symptoms in treadmill testing after repeated dosing as compared with initial dosing.^[1] Potential mechanisms of nitrate tolerance include the production of free radicals, which reduces the biotransformation of organic nitrates to nitric oxide (NO), and depletion of sulfhydryl groups involved in the conversion of nitrates to NO. In addition, counterregulatory mechanisms including volume expansion via sodium and water retention may also be involved.
Intermittent dosing of organic nitrates to provide a nitrate-free interval has been the preferred approach to nitrate tolerance for 2 decades

References:
http://www.medscape.com/viewarticle/812374