If you accidentally turn the standby/delay switch off with the unit running, D3 will usually keep the switch from seriously arcing and suffering a premature death. Most switches will eventually fail if used over their rated voltage. When you quickly stop flowing current in an inductive circuit (that includes circuits with parasitic inductance) you get a high voltage from the E = L di/dt law. This high voltage is one reason why I like to use FETs with a protection TVS for standby/ delay switches. FETs can be picky, but if designed right, they last practically forever.

When the connection to ground from the power supply is opened up to implement a "soft start" or delayed turn on, a catch diode (D3) is recommended to protect the switch. This diode is strongly recommended (possibly mandatory) in choke input supplies but is  recommended for capacitor input supplies because of the leakage inductance in the power transformer.

.

In the picture above, The switch could have a 20W sand cast resistor placed across it to pre-charge the output caps when in standby. I would pick a 10W to 20W resistor and run it at less than rated power with the full maximum CT voltage of the transformer voltage across it. That way if there is a short, there is no smoke and flames. A power line fuse won't protect the resistor in this usage.

With a 700VCT transformer, with the switch in the center leg as shown, each leg will generate 350Vrms at full load. This voltage can increase by 20% at no-load to light load and will also increase at high line. For example, using two 10W resistors (500Vpk 353Vrms) in series, the total recommended resistor value would be:

((135V high line/120V transformer rated) *700Vrms /2 * 1.2)**2 / (2 *10W) =  472.5V**2/20W = 11.16K min total resistance allowed and the resistors will need a 472Vrms voltage rating. 

5K and 10K 10W are available at Tubes and More. com we need the 10K.  Using two 10K in series (for 20K) we get 11.1W total loss or 5.5W in each 10W resistor. I'm OK with this high of a pre-charge resistor value and with the lower loss because sand cast resistors run scorching hot (275C) at rated power.