Default transitions and Place in order with transitions
Apply the default transition to many edits at once in Video by Zaweba with Ctrl+D, Shift+D or Ctrl+Shift+D, or lay files down in order with transitions.

Adding transitions one edit at a time is fine for a short film with three cuts. It gets tedious on a slideshow of forty photographs or a montage of holiday clips. Video by Zaweba has two quicker routes: a default transition you can drop on every selected edit with one key, and a bin command that lays a set of files on the timeline in order with transitions already between them.
The default transition and its three shortcuts
On the Edit menu you'll find Default transition. It applies to every selected edit, not just the one under the playhead, and there are three keyboard shortcuts depending on what you want to join.
Ctrl+D puts the default picture transition on the selected edits. Shift+D puts a sound crossfade on them instead. Ctrl+Shift+D does both, so the picture and the sound change together. That last one is the one you'll reach for most often on clips that carry their own sound, because a picture dissolve with a hard sound cut underneath tends to sound abrupt, and a sound crossfade with a hard picture cut can feel muddy.
The practical way to use it is to select the clips whose joins you want to soften, press the shortcut, and play the result. If one or two joins need something different, change those by hand afterwards. Starting from a sensible default and adjusting the exceptions is much faster than building every transition from scratch.
Place in order with transitions
The second tool lives on the media bin's right-click menu. Select several files in the bin, choose Place in order with transitions, and they go down on the timeline one after another with transitions between them. It's the quickest way to turn a folder of photographs into a slideshow or a stack of short clips into a rough first assembly.
Get the order right in the bin before you use it. Rename or sort the files so they run in the sequence you want, then place them. It's easier than dragging clips around on the timeline afterwards, especially when there are dozens of them.
Jobs these two are good for
A photo slideshow for a birthday or a leaving do is the obvious one. Place the pictures in order with transitions, put a music track underneath, and you have something watchable in a few minutes. From there you can trim the timings, add a title at the start and fix any picture that's the wrong way up.
Montages are another. A run of short clips from a day out, all with their own sound, benefits from Ctrl+Shift+D across the lot, so the wind noise and chatter crossfade along with the picture.
Interview edits are where Shift+D shines on its own. You often want hard picture cuts, because a dissolve in the middle of someone's sentence looks odd, but a tiny sound crossfade at each join removes the clicks and the jumps in room tone. Select the joins, press Shift+D, and the sound smooths out while the picture stays crisp.
Restraint with transitions
Don't put transitions on everything just because it's easy. A dissolve says time has passed or the scene has changed. If every single cut dissolves, the viewer stops reading them that way and the film feels slow. Straight cuts are the backbone of most edits, and transitions work best as punctuation.
Watch the clip handles too. A transition needs a little spare footage on each side of the join to overlap. If a clip has been trimmed right to its first or last frame, there's less to work with, so leave a few frames of slack at the ends of shots you know you'll join with a transition.
Finally, check the whole thing at full speed after a bulk change. It's easy to miss one join that landed in the wrong place, such as a crossfade over the start of a line of dialogue.
Swapping single joins later
Once the defaults are in, the transitions overview and the individual transition families in Video by Zaweba let you swap any single join for a wipe, a slide, a zoom or a smooth jump cut when a moment calls for it.